It's bunker://hex-pubkey?relay=relayurl
Every time you want to connect to a new app you new to create a new app connection in the permissions screen and then you can paste the bunker url in the web app 

 I need to search for a tutorial to convert npub to hex and then manufacture the url.
Terrible experience 😭 

 Just put a copy button on Amber! 

 There's a copy to clipboard button 

 And when you create the app it automatically copy to clipboard  

 Okay, I just find out this feature 👏. First, create a "permission app" then copy the url 👏