does anyone know how to use their ability? i click it and select my army. but i can't chose any city or other domain. where and what shall i click to make it work?

You need to chose an opponent outpost or administrative center (what an outpost become after being attached to a city). You need to have an army selected when chosing a target. Im not sure if you need your army to be in the targeted territory or not to be able to select the target, maybe thats your issue.

It cannot be used on a city

beign expansionnist, you can enter ennemy territory without open border, but doing so will give him a grievance for trespassing

You also need to be at peace iirc

I was confused too, there are other points which has to be true:

- you need to be at peace

- I think your enemy has to be in peace too (not sure)

- if you like to steal an administrative center, the attached city may not be under siege (i'm sure)

- you can't steal from allys

- you have to break the "non-aggression pact"!

if you have other facts for this particular ability, I can ad those to the list. Maybe some other ppl are confused too, bc the ability is not good enough explained ingame. as some other things too.......

Another issue I see in every game is that *any* battle involving the unit capturing of the outpost will interrupt the action. The other civ usually sends troops to stop you capturing their outpost and "protecting" your unit doing the capture from sufficiently far away (so it does not get involved) in the battle is a bit too random and frustrating imho. Even if the unit is only involved as a reinforcement and unused during the battle, just being too close to a battle and you will lose the precious gold you paid for the capture.