Considering that you have already successfully included a link in this very question, I am not sure what exactly it is that you don't know. Posts and questions are edited the same way, just like answers and comments:

Select a few words in your text; a pop-up with several buttons will appear; click the "Link" button; write (or typically paste) the URL; and click the green "Save" icon. Done.

How to obtain the link to another article? Just open it in another browser tab, then copy the URL from the URL bar.

Have you perhaps activated the Markdown editor in your settings? Again, links to other LW posts are done exactly the same as external links. (Or you can go to settings and activate the WYSIWYG editor again.)

If this didn't help, please provide more details in your question.