What is happening at the time you want to enter this value?
Have you just created a new record in tbl_Diversion and want that value entered automatically?
If so, use fileOptions to have this script performed each time the file is opened:
Go to Layout ["tbl_User" (tbl_User) ]
Enter Find Mode 
Set Field [Tbl_User::LogInName ; Get ( Accountname ) ]
Set Error Capture [on]
Perform Find 
If [Not Get ( FoundCount ) ]
Show Custom Dialog ["Error: Log In Name not found in tbl_User"]
Set Variable [$$UserID ; value: tbl_User::PK_UserID]
Then, any time you need to enter the current user's user ID into a field, you simply need to enter the value of the $$UserID global variable.
For tbl_Diversion, if you set up FK_UserID with this auto-enter calculation:
Any new record created in that table will automatically enter the current user's ID.
Worked like a champ! Thank you Phil.