I Know a Guy

Creating a living world is a monumental effort. If you understand that, then it's also safe to say that creating an entire functioning universe is even moreso.
As game masters, Administration are more than willing to work with you on your backstory and helping you weave the stories you want to tell into the mesh that we've developed over the years. That said, this isn't always necessary for minor details like a childhood friend, or a fence your character was on good terms with during their criminal days, or a world somewhere in the cosmic backdrop that makes up Realmspace.
The "I Know a Guy" rule is blanket permission for you and anyone else on the server to ad-lib your own lore (people, places, and objects) into the universe, provided you do so within reason, to support your character's development and interaction with other characters without needing immediate staff involvement. It should also go without saying but the limitations of "I Know a Guy" are expanded with Domains. As you are given greater control over what lore you can contribute to the world, you are given more freedom about what you can self-insert without needing someone to hold your hand.
You are always free to double-check in #ask-a-gan with a staff member if you're not sure whether something is too much to be considered legal or viable to insert on your own.
Examples:

"I was born to a cult trying to bring on the apocalypse. They prophesied that I would be a part of something wonderful and terrible, and held me up on a pedestal my whole life believing I would enact some grand purpose."

There is no special approval needed for something like this. More importantly, the abstract nature of the statement leaves it open ended to interpretation. There doesn't even need to be special approval for it to tie in to server-wide events, like the (as of this writing) ongoing apocalypse. These events are so large scale anyone could reasonably claim they're somehow involved in it for better or worse, and it will not effect anything but their own stories - whether it's true or not. The point here is even if it's a significant prophesy, there's plenty of room in here for plausible deniability as well.

"Hey guys, I know a guy in Senschk that's a pro with this stuff. He might be able to help us."

This is 100% within reason, and no one's going to be upset that you've done this. It doesn't even matter if Senschk isn't on a map. At this point, whoever's GMing the session is free to ask you more questions to populate something to work with. It is also entirely within reason for a GM to say this isn't something that can be pursued at the moment for one reason or another (like, say, four lairs deep in a mega dungeon), but that doesn't make any less a valid part of your backstory that could be followed up on later.

"I'm the chosen hero of Baal. He chose me specifically for my murderous qualities, and makes sure I never die in combat. All I have to do is ask for favors and I get them!"

This is an example of overstepping bounds, unless the player is detailing a character who is markedly insane - still, with or without these claims, Baal is not granting party favors to your character just because you bat your eyelashes and said you were friends with him in your backstory.

"I'm actually a secret agent working on behalf of the raven queen to steal the names of everyone I've ever met. I don't know I'm doing this, and eventually the queen will summon me back specifically to take the names of everyone I've collected and make them hers."

This is problematic on several levels. It's not out of the question to be a secret agent, nor is it a problem to work with a fey court. It is a problem to assume you're working on behalf of a known archfey, and it is a problem to say you're capable of using a form of magic reserved for GMs without even knowing what it is, and inflicting that on other players (even with their knowledge) is way beyond the scope of what a player can reasonably do or reference as part of their backstory.