It’s not advisable, but you can hardcode the username and password into the URL and bookmark that. I think we have a ticket for implementing a proper login page, yet nobody cares enough about this to implement it. If your syncthing is listening localhost only, and you are the only one using the machine, you can probably disable authentication all together.
Probably works fine with the browser built-in password managers, but for example 1Password doesn’t like our current system. I’d be happy to see it become a real login page instead.
I don’t want (and need) the prompt as I run my own authentication with Vouch Proxy. It would be nice of Syncthing would allow header (also named proxy) authentication like FileBrowser and Navidrome.