EDIT: I noticed that Reactive Counterspell is in Magic of Faerun, does that mean I have to ask my DM first because Faerun is a specific region?
It is not a region it is a "setting".
So unless you are playing in a Forgotten Realms setting game then yes you must ask permission to import a spell.
Sort of like looking at the Eberron game and importing a Kalashtar or a dragon mark feat - all setting specific.
I am surprised if your DM doesn't make you ask before bringing in new spells (i.e., those not in the PHB) in the first place. Every DM I have played with requires the player to present the new spell to him before he allows it in. Even if it is only so that the DM can "understand" what the spell does in the first place. Sometimes the DM will give a "blanket" inclusion if he already has the source and understands it.