There's no built-in "buff user" special effect type, so you'd need to add a custom one. The current buff special effect is handled in obj_battlecontrol's Step event at line 735-743, you'd copy that block to a new case in the switch statement that begins at line 780 (where effects that affect the user are handled). Just replacing the movespfx_BUFF constant in your new copy with the new constant, and changing all instances of the target variable trg to a_user in the new copy should be enough to make the code work.
Once all that is in place, just set your move's special effect to your new movespfx_BUFFUSER (and include the stat and buff tier like the existing buffs/debuffs) and you're good to go!