Your comments

My understanding is that you use the token supplied to generate an authorization URL, which you then display in a browser. The user then logs in with their username and password in order to authenticate the token, which you then use to make API calls. However, I'm using Nokia's OAuth2 library so I'm not sure about the specifics.

Under normal circumstances, the username is only entered in the form. I'm looking for a way to specify to imgur that a username has already been entered (and therefore only ask for the password), as the other approach is problematic with Nokia's framework.