The documentation you link to seems to presume that developers reading it will be writing iOS applications with Objective-C or Swift, rather than FileMaker. This doesn't mean that integrating with FileMaker won't be possible, only that you may have to experience beyond what the documentation tells you to get it to work.
What about the callback doesn't work? Does FileMaker Go re-open after Scannr scans a barcode, or not? If FileMaker Go does re-open, what parameter does the script have to work with? This latter question seems like a big question mark after reading the data. Once you're at that point, I'd start by seeing if Scannr is just appending its data to the end of the URL callback by adding "¶m=" to the end of the URL:
The hope is that the data returned from Scannr will just be concatenated to the end of the URL.
If Scannr or some other iOS app doesn't work out, getting a hardware scanner might be easier to integrate, albeit more expensive.