https://www.youtube.com/user/marquesbrownlee

You should see a blue „Join“ button next to the sub button.

It‘s called channel membership. You need to be in the partner program and have 30‘000 subscribers (1000 for gaming channels) to activate it.

https://creatoracademy.youtube.com/page/course/channel-membe...

Edit: depending on what your region is it might not be a thing yet. But if your region supports Premium, I‘m pretty sure memberships are supported as well.

There's a "join" button next to the "subscribe" button. You have to be logged in to see it.