need help with how to specify source entity

so, i'm trying to make a mod that has an effect that on tick triggers the possibility of taming an mob so you can only tame it while it has the effect, but it needs the event/target entity and the source entity, being event/target entity the mob and being source entity the player, but here is the problem, don't know how to specify both entities, if someone know how to it would be helpful (i'm on version 2023.4 of MCreator)