I'm going to post the general idea, but someone else can be more specific (right now I'm tired). Make an entity, link to it a trigger_teleport, create a misc_teleporter_dest, and link it to the trigger_teleport. The entity you made should be invisible in-game, so if you want players to see the teleporter, you'll have to make other entities around it. The area where players should trigger the teleporter should be as large as the entity to which the trigger_teleport is linked.