By portal, do you mean moongate? If so, create a nonlooping patrol that has 1 point on the entrance moongate, 1 point on the exit moongate.

Create a script with the trigger = "Faction Change". Entities = select a single tile in the room (don't select all the tiles in the room or the script will trigger multiple times when the room is claimed). Set the faction to the player faction (ie. underlord1).

Create an action that "assigns patrol" to the units you want to use the moongate. You can also spawn units at the entrance moongate and tell them to use the patrol (rather than having idle units on the map that's only purpose is to use the moongate).

Another option is to not have a patrol and have a moongate/other asset that you spawn units on (to make it look like they used the moongate) or you can also use "teleport entity" to move a unit to the exit moongate. However you cannot teleport a unit to the entrance moongate since they won't use it.

I have a similar map that spawns units when the player enters a keep. I use the "forcefield door" (only lets friend units through) as a "portal art" and spawn units directly onto the tile to simulate them "coming through the portal door".