What would be better would be to make a value list of existing IDs and validate the field to only allow entry of a valid ID. Having someone manually type in keys is just asking for trouble.
That said, a simple function like this:
Not IsEmpty (CLIENTS::CLIENTpartner_id) and Count(CLIENTS_partners::CLIENTid) = 0
what you're doing is letting a user input a key. Go to the blackboard and write 100 times "Ill never let a user input a record key"