Capturing User Name as Variable

Created a web site using Fusion 8, hosted by 1and1 ISP.
Directories are user name/password protected at ISP.

When URL is typed in , a pop-up will authenticate to get user name/password 
since directories are protected.

How do I capture the user name entered, store it in a variable, so I can use 
it to control page behavior after authentication  ? (i.e. gray out buttons, 
restrict access to selected sections in the site, etc.)

Thanks