The prefilled message can be edited
The message arrives as a draft, and the sender can change or delete any of it before hitting send. That matters because you cannot assume the text you prefilled is the text that reaches you.
So never rely on an SMS QR for anything that depends on exact wording — verification codes, payment confirmations, identity checks. Whatever lands in your inbox may be whatever the sender decided to type instead. Treat the prefilled message purely as a convenience that saves the other person some typing.
Some devices fill only the number
The code carries both the recipient number and the message body, but what happens with them is up to the scanning device and its messaging app. Usually both get filled in, though on some devices and apps the message screen opens with the number set and the body empty.
That means nothing essential should live only in the message. Text like "Could you please move your car?" works because the recipient can infer the situation without it — and shorter is better anyway. Once you've made the code, scan it once on Android and once on iPhone.
Store the recipient number in international form
The number here is the one that will receive the text. Type it with hyphens or spaces and the generator keeps only the digits, so for a code shared between people in the same country your usual format works fine.
For someone scanning it in another country to text you, the number has to be reachable internationally: write it with the country code and no leading zero, as in +82 10 1234 5678, so their carrier routes the reply correctly. The generator leaves only a leading + as you typed it, which means removing that zero is something you do when entering the number.