If you connect the scanner to the phone by HID mode, the scanner will act as a keyboard to the phone. When it scans a barcode, it will send the barcode to the currently active program just as you are typing a number with the phone’s keyboard.
To verify your scanner is indeed paired to the phone successfully, you can open your Email app on your phone and see if you can scan the barcode to your Email app. If you can scan the barcodes to your Email app, then your scanner is paired successfully and you should be able to scan the barcode to the FBAScan app as well.
Here is a video show you how to do this.