I need an addition to a filemaker pro 12 database

I need someone to add a barcode form to our inventory system. Our setup is the exact same as the inventory starter solution on filemaker 12. I would like to be able to scan a barcode that indicates ADD Quantity ( Can be any string) and then start scanning barcodes that correspond to the barcode field for our items. For each time I scan the barcode I want the quantity to increase by one. I would then like to be able to scan a barcode that indicates DELETE Quantity and the start scanning barcodes that correspond to the barcode field for our times. For each time I scan the bar code I want the quantity to decrease by one. Our barcode scanner simply emulates a keyboard and appends a CR after each barcode string. So we would just need I guess a text box we leave the computer's cursor in at all times to accept the data.