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?

I originally did what SMeznaric suggested after changing the Investiture law in my primary Empire title. The problem was that the button was greyed out with the same message regardless of what the investiture law. If you have a well-loved Pope, no one will stand up to him regardless of the law.

Summary: The lowest de jure crown laws of the bishopric seem to apply. This seems consistent with the wiki description:

Crown laws always apply within their de jure provinces, even if those provinces are part of another realm. Thus, if you conquer territory in another de jure kingdom, and there is a king of that kingdom, their crown laws will apply to those holdings. If parts of your realm belong to unincorporated kingdoms, the crown laws of your primary title apply instead.

So I played a little bit more and low and behold, his Most Popular Excellency of the 14th Century died stressed and ill at the ripe old age of 47. His successor is Pope Not-So-Great XIV. It turns out, I don't even need to change my investiture law to appoint an Antipope! There are vassal bishops under other de jure realms which do have free investiture and I can appoint them as Antipope.

For example, I hold the county of Brugge (with its 7 delicious holding spots). It is de jure part of the Duchy of Flanders, Kingdom of Frisia (nonexistent) and the Holy Roman Empire. Flanders is vassal to Holy Rome, so the crown laws of the HRE apply. The Kaiser, as it happens, has his own antipope and is about to lose a war to have him deposed, which I think will cause him to switch to papal investiture. As long as I appoint my antipope before that happens, I think I'll be golden.

Also, my vassal bishops (actually archbishops, holding duchy titles) in the Kingdom of Africa are free to become antipopes, because Africa does not have an investiture law, probably due to a bug from usurping the title from a Muslim ruler.

I have other bishops who are under the Empire and Kingdom titles I control: changing just the Kingdom Investiture allowed me to appoint them as antipope. I did not check whether changing the Empire Investiture law impacted these bishops or not.

I checked around a bit for a bishop who is only under my empire title with no other ducal or kingdom titles existing between him and me and didn't find one. In other words, I can't confirm that only the Empire crown laws would apply to him.

Also not tested was which crown laws allow or block an independent duke appointing an antipope.

All tests and screenshots are from the current version of the game, 2.4.2. DLC's active for this savegame are Sons of Abraham, Way of Life and Ruler Designer.