It would go to the best available charity that is working to fight that particular existential risk. For example, the 'donate' button for hostile AI might go to MIRI. The donate button for pandemics might go the Center for Disease Control, and the donate button for nuclear holocaust might go to the Global Threat Reduction Initiative. If we can't agree on which agency is best for a particular risk, we can pick one at random from the front-runners.

If you have ideas for which charities are the best for a particular risk, please share them here! That is part of the work that needs to get done.