The user takes the target trick-or-treating. This adds Ghost type to the target's type.
Trick-or-Treat adds the Ghost type to the target, in addition to the Pokémon's original type(s).
If it is used against a dual-type Pokémon, that Pokémon will have three types at once. If the target already has an additional type added to it by Forest's Curse, that type is replaced with the Ghost type.
Trick-or-Treat will fail if the target Pokémon is already Ghost-type.