Which Realm's crown laws apply for creating an antipope?

I finally have an empire large enough to rival the papacy. I want to create an Antipope, but I've hit a snag. I changed the investiture law in my Empire, but was not able to appoint any bishops as the antipope.

I discovered later that all of my bishops have a +100 opinion of the current Zealous, Crusading, Charitable pope, so that could be a part of why the diplomacy option doesn't appear. This can be seen from the religion interface, which is where you need to go to actually make an antipope.

^{Screenshot from game version 2.4.2}

This made me wonder, which crown law or laws apply for the condition "the realm is under Free investiture law"?

For example, I am the Emperor of Britannia, and I hold the Kingdom of England title as well. If I want to appoint the bishop of Canterbury an antipope, does the Empire or Kingdom or both have to have Free Investiture?