You could use .lua Script
Code:
function Undercity_Grom (pUnit, Event)
pUnit:Teleport (MapID, X, Y, Z)
end
RegisterGameObjectEvent (200017, 2, "Undercity_Grom")
Open note pad and put the Above in it
Ok first > Undercity_Grom
Change that to were your portal is going to go, eg "Stormwind"
Ok Second > MapID, X, Y, Z
Put the Mapid x, y, z position of were you wont the portal to port you
Ok 3 > 200017
Put the number of your Gameobject there
Ok last > Undercity_Grom
Change that to were your portal is going to go, eg "Stormwind"
The File > Save as > "Name of Portal.lua"