YouTube offers a valuable service for partners, which is Fan Finder.

https://www.youtube.com/yt/fanfinder/

You need to make sure your channel has a short video, less than two minutes and ideally under one minute, that is copyright free and suitable for an ad. Or make one as I did below, either using material you have or new material.

They then play this ad--like an unused billboard in a way. They also provide click through to allow ppl to subscribe to your channel.

I made a short one to test it it, and sure enough, lots of clicks and an increase in subscribers. Note that if you make your vid 59 seconds long, as I did, YouTube counts this as 1:01. Still way under two minutes, but if one minute is a threshold, factor that in. For two minutes, shave the two seconds off for sure.

If the copyright trolls flag your vid, you will have to clear it through the appeals process before using it for Fan Finder. Since that can take a month, or more, figure that in to your timeline.

For this vid, I just took three clips and added a logo. I'll post figures in a few months to show how well the vid does with the free ads, and see if different ads do better.