I don't use the Perform Find function with built-in details much, but here's a way to get where you're trying to go without "defining the Find".
SetVariable [ $Clientname; Value:Client::Client Name]
Enter Find Mode
SetField [ Client::ClientName ; $ClientName ]
You can also error trap the results if you like (Set ErrorCapture on and test for what errors show up during the perform find step like 'If (Get (FoundCount) = 0 )...show custom dialog...etc.)
Ninja, you're a legend. Much obliged. :)