The thing that the God of Wind had snuck into the Mark of God was…
A contract with the Fenrir.
Since the Fenrir was the messenger of the God of Winds, it meant that there was already a contract that was formed between the Fenrir and the God of Winds. That was how the Fenrir was able to serve as the messenger of the God of Winds.
Normally this kind of contract wasn't something that could be passed onto others, but gods didn't really follow rules.
As long as gods wanted to do something, they would be able to find a way to do it.
In particular, what the God of Winds did was put the contract that it had formed with the Fenrir into the Mark of God. That meant that whoever had the Mark of God bestowed by the God of Winds would be able to become the master of the Fenrir.
This was something that was quite dangerous since it meant that the Fenrir would be at the mercy of this person, but it wasn't as if the God of Winds would give their Mark of God to just anyone.
