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.