How to get custom thumbnails without being verified?

I think that's the right tag, anyways um I tried asking this in another subreddit but got nothing, so I'm kinda wondering how do you make custom thumbnails for your videos if you're not verified, I don't have enough subs to be verified and I'm going to at some point soon really need to be able to have custom thumbnails for that, and I know it's possible considering I've seen non-verified channels have custom thumbnails so I'm just wondering how I could do that as well, so if you know how could you tell me, please and thank you.