I assume you meant app_eui, not app_key? (AppKey can be anything, and preferably is different for each device. AppEUI must be one of the AppEUIs known to the application test4849.)
Fun fact, and maybe the solution: the limited payload I linked to earlier also uses POST, but even then also uses the device ID in the URL. I’d only expect that for PUT (when updating a device), but not for POST (when creating a device). Also, the documentation you linked to, does not use the device ID in the URL. But it might not hurt to try…?
(When that does not help, then that might actually be the cause of the error in that topic I linked to…)