In this case, an <inputfield> can be added for the username. It will let the client show a textfield to the user. The user-visible label is the one provided in the <inputfield label=""> attribute, to allow the ISP to use custom terminology for "username". The label is not localizable, that would be too complicated and most ISPs are local anyways - if really important, the ISP config server can look at the HTTP headers during the fetch.
The text that the user entered is written into a placeholder %key%, where key is the content of the <inputfield key=""> attributeand must be only upper case letters. The placeholder can then be used in other settings, specifically the IMAP/POP and SMTP username.
For example, a config file entry