How can I prevent the YouTube aplication from constantly showing a pop-up for YouTube premium trial? How can I prevent the YouTube aplication from constantly showing a pop-up for YouTube premium trial? I'd like to never have this window.
I have never been disturbed with these popups so far in the modded YouTube app named YouTube Vanced so I suggest you try this app. The app also has background play feature that can be used without a YouTube Premium subscription.

Head over the XDA thread here to know more about this app and learn about its installation. 

Note: the app automatically blocks ads served by YouTube.