How to Prepare Your CSV for Clay's Exclude Companies Feature

Hey Support, if I have a csv of existing customers that I'd like excluded when doing a company search, what do I need to do to the table in Clay in order for it to be recognized by the new "Exclude Companies" feature? Seems like it can only exclude companies if I've used Clay to find those companies. I'm trying to figure out how I get my list of companies available for exclusion.