This seems to be a very loose paraphrase of Sartre drawn from the Squashed Philosophy paraphrase of "Existentialism is a Humanism":

What, then, is "existentialism"? Existentialists, be they Christians or atheists (like myself) believe that existence comes before essence- that we must begin with the subjective. A manufactured object, say a paper knife, is made to serve a purpose. For us, our existence comes first, we have each our own Project; to choose our own purpose, and in doing so to choose an image for all to follow. Existentialists state frankly that man is in anguish unable but to choose. Like Abraham, who obeyed the angel, but had to first choose if it was really an angel, and if he was really Abraham. We are abandoned by God, left alone without excuse, to interpret alone such signs as there are. Even when we seek advice, it is we who choose who to ask. We must despair of help from outside our will, or beyond the possibilities of action. Descartes said "Conquer yourself" and meant the same - that we should act without hope.

The corresponding (and far less punchier and quotable) parts from the original, using the Mairet 1956 translation, seem to be:

...If one considers an article of manufacture as, for example, a book or a paper-knife – one sees that it has been made by an artisan who had a conception of it; and he has paid attention, equally, to the conception of a paper-knife and to the pre-existent technique of production which is a part of that conception and is, at bottom, a formula. Thus the paper-knife is at the same time an article producible in a certain manner and one which, on the other hand, serves a definite purpose, for one cannot suppose that a man would produce a paper-knife without knowing what it was for. Let us say, then, of the paperknife that its essence – that is to say the sum of the formulae and the qualities which made its production and its definition possible – precedes its existence. The presence of such-and-such a paper-knife or book is thus determined before my eyes. Here, then, we are viewing the world from a technical standpoint, and we can say that production precedes existence.

...And when we speak of “abandonment” – a favorite word of Heidegger – we only mean to say that God does not exist, and that it is necessary to draw the consequences of his absence right to the end...The existentialist, on the contrary, finds it extremely embarrassing that God does not exist, for there disappears with Him all possibility of finding values in an intelligible heaven. There can no longer be any good a priori, since there is no infinite and perfect consciousness to think it. It is nowhere written that “the good” exists, that one must be honest or must not lie, since we are now upon the plane where there are only men. Dostoevsky once wrote: “If God did not exist, everything would be permitted”; and that, for existentialism, is the starting point. Everything is indeed permitted if God does not exist, and man is in consequence forlorn, for he cannot find anything to depend upon either within or outside himself. He discovers forthwith, that he is without excuse. For if indeed existence precedes essence, one will never be able to explain one’s action by reference to a given and specific human nature; in other words, there is no determinism – man is free, man is freedom. Nor, on the other hand, if God does not exist, are we provided with any values or commands that could legitimise our behaviour. Thus we have neither behind us, nor before us in a luminous realm of values, any means of justification or excuse. – We are left alone, without excuse. That is what I mean when I say that man is condemned to be free. Condemned, because he did not create himself, yet is nevertheless at liberty, and from the moment that he is thrown into this world he is responsible for everything he does.

...feeling is formed by the deeds that one does; therefore I cannot consult it as a guide to action. And that is to say that I can neither seek within myself for an authentic impulse to action, nor can I expect, from some ethic, formulae that will enable me to act. You may say that the youth did, at least, go to a professor to ask for advice. But if you seek counsel – from a priest, for example you have selected that priest; and at bottom you already knew, more or less, what he would advise. In other words, to choose an adviser is nevertheless to commit oneself by that choice. If you are a Christian, you will say, consult a priest; but there are collaborationists, priests who are resisters and priests who wait for the tide to turn: which will you choose? Had this young man chosen a priest of the resistance, or one of the collaboration, he would have decided beforehand the kind of advice he was to receive. Similarly, in coming to me, he knew what advice I should give him, and I had but one reply to make. You are free, therefore choose, that is to say, invent. No rule of general morality can show you what you ought to do: no signs are vouchsafed in this world. The Catholics will reply, “Oh, but they are!” Very well; still, it is I myself, in every case, who have to interpret the signs. While I was imprisoned, I made the acquaintance of a somewhat remarkable man, a Jesuit, who had become a member of that order in the following manner. In his life he had suffered a succession of rather severe setbacks. His father had died when he was a child, leaving him in poverty, and he had been awarded a free scholarship in a religious institution, where he had been made continually to feel that he was accepted for charity’s sake, and, in consequence, he had been denied several of those distinctions and honours which gratify children. Later, about the age of eighteen, he came to grief in a sentimental affair; and finally, at twenty-two – this was a trifle in itself, but it was the last drop that overflowed his cup – he failed in his military examination. This young man, then, could regard himself as a total failure: it was a sign – but a sign of what? He might have taken refuge in bitterness or despair. But he took it – very cleverly for him – as a sign that he was not intended for secular success, and that only the attainments of religion, those of sanctity and of faith, were accessible to him. He interpreted his record as a message from God, and became a member of the Order. Who can doubt but that this decision as to the meaning of the sign was his, and his alone? One could have drawn quite different conclusions from such a series of reverses – as, for example, that he had better become a carpenter or a revolutionary. For the decipherment of the sign, however, he bears the entire responsibility. That is what “abandonment” implies, that we ourselves decide our being. And with this abandonment goes anguish.

...As for “despair,” the meaning of this expression is extremely simple. It merely means that we limit ourselves to a reliance upon that which is within our wills, or within the sum of the probabilities which render our action feasible. Whenever one wills anything, there are always these elements of probability. If I am counting upon a visit from a friend, who may be coming by train or by tram, I presuppose that the train will arrive at the appointed time, or that the tram will not be derailed. I remain in the realm of possibilities; but one does not rely upon any possibilities beyond those that are strictly concerned in one’s action. Beyond the point at which the possibilities under consideration cease to affect my action, I ought to disinterest myself. For there is no God and no prevenient design, which can adapt the world and all its possibilities to my will. When Descartes said, “Conquer yourself rather than the world,” what he meant was, at bottom, the same – that we should act without hope.