Despite the title bar of the window, this is actually asking for an HTTPS username, not an SSH username, if you look at the URL. This is because Git is using the
ssh-askpass command to prompt for a username and password for HTTPS.
Git for Windows configures this behavior by default because many users on Windows use graphical programs to invoke Git, not a terminal. Therefore, trying to prompt on the terminal would silently fail for many users without a helpful error message.
ssh-askpass is already shipped with Git for Windows and can be used for prompting in a graphical way without any modifications to Git.
If you'd rather be prompted on the terminal, you can unset the
GIT_ASKPASS environment variable, and Git will prompt you at the terminal instead. Additionally, if you use a credential manager, Git will store your username and password for you so you need not enter it a second time, and then you won't be prompted at all.