How to send login credentials to popup window

问题:

When logging-in automatically, the username and password are usually sent to the form via POST or GET functions. However, I'm trying to automate the login for a third-party site (namely an IP camera resource) which pops-up the login forms like these:

in Chrome:

in FireFox:

in IE 11:

The dev tools of Chrome, Firefox and IE don't seem to be able to inspect the popup. How would we obtain the form's input names, and how could we send it to the form.

Thank you.