Substitutions rule for updating customer # on GL accounts

Hi Gurus

I am trying to use substitution rule to update the customer field (BSEG-VPTNR) with customer number from (BSEG-KUNNR) the first line item.

So I am trying write the following substitute logic:

Prerequisite: BSEG-KUNNR <> '' AND BSEG-VPTNR = ''

Substitutions: user exist: and it seems this this logic didn’t work. Kindly guide me with setting up the substitutions:

Thanks