I think I have found another solution.
The user logs to my website. I use this endpoint:
Thanks to it, I return the subscription that user have.
If the subscription match with my channel ID, then I’m done.
Is it good?
So I can use implicit oauth only.
Essentially, I was returning the subscribers on my channel and then check if a certain user ID was inside.
But I can reverse and say: the user logs in, I get his subscriptions, I check if my channel-Id is inside there.