The OnWaveSpawnedEvent is actually an Event Dispatcher Event from an Object (Brewery5) in the Level.
To create a new WaveSpawner, dublicate one of the Brewery. Then select the new one and go into your level blueprint. Right click somewhere and you’ll see that you can “Add Event from Brewery”. There you’ll find “OnWaveSpawned” 
If you want to add functionality to the Spawn, you need to extend the existing OnWaveSpawned Event in your Level Blueprint.
You can only delegate an Event once in your Level Blueprint.