if you don't want to or can't use extensions, just right click > inspect on the password field, then right click the element highlighted in the HTML and click "use in console" or "store in global variable" depending on browser
it'll put something like
temp0 into the console
just change that line to
temp0.value = "yourpassword"
``` and press enter
it sounds verbose to explain but it's just a couple of clicks and one command, if you're using a password manager it's still a lot easier than typing out a random string and it should work with most text boxes and inputs, might not work if the page is doing something fancy.
Tbf Steam also messes around with defaults like making my controller's guide button focus Steam instead of opening the game bar - which is maddening in the middle of a game. I'm a couch gamer so I have to go grab my keyboard or whatever in order to Alt+Tab back to the game.
It also by default remaps to use the Steam inputs driver.
I swear I keep turning these settings off and then mysteriously they re-enable sometimes. It's mostly rigged up so that as long as you're only launching games through Steam it works great but not otherwise.