Make a new interaction that does both the material emission and the syndrome effect.
But what about NPCs? USAGE_HINT:ATTACK doesn't work with TARGET:SELF_ONLY.
Make the spell do both an untargeted and targeted material emission, untargeted transmits the anti-magic syndrome. Not 100% reliable.
Or if nobody comes up with something smarter, you can do bouncing syndromes which let you do more complex interactions between two creatures. Something like:
Eat magic pie
get syn CAN_INITIATE_MAGIC_TRANSACTION.
CAN_INITIATE_MAGIC_TRANSACTION: add tag: MISCHIEVIOUS. cdi: free action: attack: MAGIC_RECIPROCITY.
MAGIC_RECIPROCITY: adds syn CAN_RECIPROCATE_MAGIC to enemy.
CAN_RECIPROCATE_MAGIC: cdi: RECIPROCAL_MAGIC_ENABLE: attack, free action. RECIPROCAL_MAGIC_ENABLE: target requires: MISCHIEVIOUS. Adds syn MAGIC_ENABLED to target.
MAGIC_ENABLED: cdi: MAGIC_SPELL: attack (100 phases, usage delay 200 phases). cdi: MAGIC_FINISH (start: 100 phases, end: 300 phases?) target: self only; no usage hint.
MAGIC_FINISH: add syndrome: remove tag. MISCHIEVIOUS
Would not work for the player character since you're just not going to want to do all those interactions yourself every time. The player character should instead get an interaction that gives them a syndrome that first enables magic for a short while and then burns it out. NPCs wouldn't know how to use that properly.