Forward Posts act as spawn points for both attackers and defenders in Isonzo, as well as offering call-in abilities for the Officer class, and are highlighted on your mini-map as flag icons. Blue flags mean they're captured by your team, red flags mean they're captured by the enemy (these can be destroyed by any class by holding X on the Post when you find it), and white flags mean the point isn't owned by anyone.
Once you've found a neutral
Forward Post, it can be spotted easily enough thanks to the bright green highlighting it has in the shape of an outbuilding of sorts. Walk over to it and
hold X as any class (Engineers are generally the quickest) and after a short time, you'll have it built.
With that, your achievement should pop! 💀🐺