In your system startup file (for V6.2, this file is
SYS$MANAGER:SYSTARTUP_VMS.COM), you may include a definition of the logical
$ DEFINE /SYSTEM SYS$ANNOUNCE "@SYS$MANAGER:ANNOUNCE.TXT"
Whatever text you include in the file SYS$MANAGER:ANNOUNCE.TXT will then be
displayed prior to the "Username: " prompt.
This logical is documented in the system startup template file