I know as of this moment Flarum still doesn't have built-in user management console. or anything relate to users setting at all
But I want to suggest this. Custom Profile Field with Callback support.
It's something like when user save the field. It will callback to something like Amazon Lambda or Google Cloud Function to process filled data and return simple value back to Flarum
This will make it easier to make verification like on support forum which required product key to access something.