Trying to Detect On Attacker with the damage command.

Sorry if the title is confusing, but I'm trying to make it apply a tag to an attacker that is specified in a /damage command. For example, if a mob would attack me and pop my totem, they would gain a tag to eventually be killed, but I'm wondering why it doesn't work when the enemy is damaging the player when they're specified in a /damage command, like: /damage [Player] 999 minecraft:mob_attack [Mob]. But it doesn't seem to register the mob as the attacker for the command applying the tag.