When you are on a page with the text quoted below, you need to enter your login:

Login
Enter your username for our site to use with your preferred OAuth/OpeniD service.
Username: ***
A CAUTION: The Username that you choose to login with will be displayed to everyone. It is strongly recommended to not attempt to use an email address.
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However, if you don't remember your login on the site selected in the second field, you will have to launch your password manager every time. It would be much easier to make the login field in such a way that the browser would treat it as a password, and allow it to remember the value entered into it, and offer it every time your site requires it.

Mozilla Firefox 135.0b6 (64-bit)