You have to file your application for possession directly at the court of the demi-god in question. Office hours are workdays 9-12, bring your own sacrifice.

No smoking in the office, except for the one in Malfeas. Wyrm very much likes smoking. It also has several branch offices in the Labyrinth, close to oblivion etc. The other providers of possession are more discerning when it comes to applicants. Please avoid blood stains on your CV.

Haha very funny, so I assume I just explore an umbra within their respective domains and Ill get the option to become possessed as a random encounter?

I'm also assuming possession options are exclusive to werewolf splats

Yeah, plus a vibe check I guess. They are a bit picky at times.

Wolves and cats are quite flexible. Fanatics and dead things not so much.

Garou are already possessed by a Gaia spirit, still they get a second one if they ask real nice.
Fae have a lot of spirit, still they may get even more.
Do the damned get nothing? Not quite. Yet the most eager patron is not the most rewarding.
The evil fae queen gives a lot and asks for very little in return. Why is she evil again?

Well i managed to get my silver fang werewolf possessed. Are vampires not able to be possessed?

They can be possessed by a number of entities. Usually it is not good for them.
Can they become Kami by having a nice chat with Gaia? Nope.

引用自 random encounter：

Garou are already possessed by a Gaia spirit, still they get a second one if they ask real nice.
Fae have a lot of spirit, still they may get even more.
Do the damned get nothing? Not quite. Yet the most eager patron is not the most rewarding.
The evil fae queen gives a lot and asks for very little in return. Why is she evil again?

Pretty sure Mummies can also be possessed, as I remember seeing something about "Bane Mummies" at one point.

Yeah, a version earlier I may have gone kami with the black hand mummy. For some reason one of the kami abilities is supremely nasty. You warpbomb your enemies to Yomi. Sounds neutral good to me. Mummies are more immortal than undead. It is somewhat plausible that Gaia might stoop to talking to them.
Bane mummies are not really possessed per se. They were turned by the cheap knockoff version of the spell of life that Seth could afford. It´s like the Horus original - just a little crappy and replete with side effects. If Gaia also talks to them I would consider that to be a major oversight by the devs. Wyrm might, though.