If by "prefill UserName" you mean you have to initialize the string variable that the GetUserNameW API function returns, then Yes, you MUST perform this initialization, as Roland showed in the code snippet he provided.

Any buffer passed to a Windows API function that gets assigned a value by the API function must always first be initialized by the PB application to its maximum length before the API function is called. Typically, this is easily done in PB via the Space PowerScript function, as Roland's code shows.

In this particular case, the GetUserNameW API expects the buffer to be 257 characters long (256 usable characters plus a string-terminating null character). The UNLEN (User Name LENgth) constant has the value 256, and the code adds 1 for the terminating null. This is clearly described in the Windows API documentation for the GetUserNameW function:

https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getusernamew

As stated in the above documentation link, the UNLEN constant is defined in the C/C++ header file named lmcons.h. This is where Roland determined the proper value for this constant.

I got the same error, when the ls_username was initialized with spaces or blank