In the latest versions of MaNGOS you can't create portals using areatrigger_template anymore. What you will need to do is create a gameobject that will use a teleporting spell with chosen coordinates.
Step 1 - Finding a teleport spell
Go to Wowhead, search for teleport.
Go to the category: Uncategorized spells
Look around for spells with the effect type: "Effect: Teleport units" (screenshot)
Some example spells you can use:
29216
20618
20682
34673
There are alot more you can use, just look around.
Step 2 - Adding coordinates for the spell
Open table: spell_target_position in your mangos database (Using any SQL GUI, eg. navicat).
Create a new row, and put the choosen spell ID in the 'id' column.
Use the .gps command ingame at the teleport position you want for the portal and copy the coordinates to the position columns in the table.
(it will say the Map ID in .gps so please use that!)
The selected coordinates in the screenshot are the ones you need, ignore the rest.