Implementing bar code scanner

You will need to talk to the manufacturer of your scanner.

Most scanners are shipped set to emulate a keyboard - so when you scan the code, the barcode data is transferred as if it had been typed at the keyboard. And Windows does not keep any track of the source of keyboard data, if there are two or more keyboard devices (or keyboard emulating devices, Windows doesn't care) then the data is fed into the same keyboard queue in the order they are received by the system.
Which means that you cannot tell if any input came from the scanner or from a user typing, and so you can't direct it at any particular control.

However, most scanners can be configured to give specific "lead in" and "tail out" sequences that you can detect, and direct the data between them where you want.

But ... each manufacturer - and often each model - has a different way to configure it, so you will need to contact the manufacturer and find out how to set the sequences.