How do the thin client get the Windows username?

The most reliable way to retrieve this kind of info is by calling Windows functions.

The API function GetUserName does the trick.

Please give the following code a try:

       program-id. prg.

       working-storage section.     
       77   userName pic x(32).      
       77   len      pic 99 value 32.
     
       procedure division.
       main.
           call client "C$SETENV" using "dll_convention", "1".
           call client "GetUserNameA" using by reference userName
                                            by reference len
              on exception
                 call client "ADVAPI32.dll"                                            
                 call client "GetUserNameA" using by reference userName
                                            by reference len
           end-call.
           display userName.
           goback.