What made Trademeet apear on the world map?

Casters

Areas I haven't visited are highlighted in blue, like Trademeet. So far locations aren't revealed on the world map unless I talk to a certain NPC or read an item that tells me of the place. I don't remember how Trademeet was added, or if it was already there to begin with.

Grond0

Talking to someone in the City Gates area gives you that quest.

megamike15

someone at the gate district tells you.

TheArtisan

I believe Mazzy also puts the area on your map when you kick her out.

Gallowglass

Or Trademeet can also be added to the map by Meronda (whom you meet in Neera's quest, if you've started that before visiting Trademeet).

But most likely, you spoke to Flydian in the City Gates district.