Philip99 You could also use checks for a random UID (unique ID) or IID (instance ID; if you're spawning objects and run the check inside the condition they're generated, use this). If you're spawning multiples of one sprite, you can try:
System -> On start of layout
- (Sub-event) family_enemies -> Pick instance with UID (random(1,maxNumOfSpawnsHere))
---- Add events here for your power-up to apply to this picked instance.
If running the power-up check inside the main condition where the sprites are spawned, use IID and place the check for IID as a sub-event somewhere after spawning.