I just tried it. Installed my.openHAB addon via Paper UI.
The file userdata/uuid is creates in any case and once the addon is there, I can also see userdata/myopenhab/secret.
Entered both in my my.openHAB account and restarted my runtime I see
21:26:49.605 [INFO ] [o.myopenhab.internal.MyOpenHABClient] - Connected to my.openHAB service (UUID = c77049de-e5d0-45ab-94c6-xxxxxxxxx, base URL = http://localhost:8080)
Did you maybe not look in these locations for the files?