What about a “Generate uplink ID” button on the app that says “Please download this file and save it on the same folder with the uplink”. The button will generate a text file with an unique ID and offer to download it.
The user will save that file on the uplink folder. At this point the uplink will read the ID from that file. If the file is not there, it will ask the user to go to the app and generate one.
EDIT
Some message should be printed by both uplink and “Generate uplink ID” button to make it clear that the id file shouldn’t be copied to another computer.
… but at the end of the day, this is very similar to @david.wylie solution where the uplink generates the ID and you need to copy it to the form.