What bit are you struggling with?

I assume you already know how to create a public method, thats basic C++.
The contents of that method - assigning parameters to class member variables, is also basic C++.

So the only part left is the actual call, and so you simply instantiate a copy of the login form from within your main window (either allocate it on the stack and call a function which doesn't return until the login dialog is closed, or use the 'new' operator to allocate from the heap)