If you mean engine unlock of the class, you need a DM to do it for you, as with any other faction class.
ICly, though, it needs to be a part of your character's bio (past or present, so what you have mentioned actually works) and be posted as a request.
Skymage needs to be unlocked by a DM. You'll need to post a request in
this forum for that. The Zhentarim faction is currently inactive, according to
the current factions and contacts list, but there's info there on how to get in touch with someone who can help you out. Hope you enjoy playing here!
Also I can't seem to find any sort of format or template for the applications unlike on other servers, not too sure what to do about all this.
The forum is labeled Prestige Class Requests, linked above. I don't think there are any active Zhent players at the moment, but the thread I linked should have contact info about the faction if you want to see about getting it active.
Thank you for your time, will do so now, trying to contact the other Zhents and do the request about becoming a Skymage.