I'm trying to register the plugin XYZ but I cannot seem to use my keyboard to type my name and license code.

This is because your host uses your keyboard for shortcut functions and every keystroke is directed to the main window, not to the plugin's window that is on the foreground. You should check with your host's user manual, there should be an option somewhere that allows you to choose wether to route keystrokes to the foreground windows or to the main window.