Popup is quite a generic term, because you can have popup’s that are OS level dialogues created by the browser and popup’s that are HTML elements that look like dialogues.
I suggest you use the developer tools of the browser and try to inspect the user name field, if you can inspect it and see it’s an <input type html element then entering the username and password is the same as any other field, otherwise it’s probably an OS level dialogue.
If your still not sure show some screen shots of the popup and might be able to figure it out.
Dave.