Paste your channel link, @handle or channel ID.
Find it in YouTube Studio under Customization, then Basic info.
Handles (@name) and channel IDs (UC followed by 22 characters) are the most reliable. A channel ID never changes, even if you change your handle.
The box over your channel page.
https://www.youtube.com/@yourchannel?sub_confirmation=1
Ready. Signed-in viewers who don't subscribe yet see a confirm box over your channel page.