What do I do with Reef-Talon's Keystone? (spoiler)

[Dsingis]

After I found reef-talon in the old walls, I convinced her to go deeper into the old walls.
She gave me her keystone for one of the teleporters, but I can't seem to find any not activated teleporter to fit that in.

What do I do with that keystone? Where can I use it?

 

[Franconian]

It is for the teleporter in the north west, near the smithy