Requirements for tagging a mob?
Try ctrl+t this target an enemy
There is no mob tagging in Gw2. You do not “own” the mob you tagged. You need to do even the slightest dmg to be counted towards the loot.
You just have to deal some damage to it. As long as through some of action of yours that a mob is hurt then you get exp and the drop. Even if it was your pet or minion that dealt the damage. Doesn’t matter how small the damage was.