The script step you are looking for is called Omit Record. It works to turn on the Omit function in a find request.
Try a script like this:
Enter Find Mode [ ]
Show Custom Dialog [ Title: "Found It"; Message: "Enter the serial here."; Buttons: “Omit”, “Stop”; Input #1: Data::ID,
Exit Loop If [ Get ( LastMessageChoice ) = 2 // Cancel ]
Constrain Found Set [ ]
Enter Browse Mode
See how that goes for you. And it doesn't need a global field as you were using.
TS_Oz, FileMaker Inc.