How does the Moving Platform work? How are you moving it?
If the whole Actor moves, You should be able to drag the Bonus BP on top of the Moving Platform on the World Outliner and it will get attached.
If you’re only moving the mesh inside the Moving Platform, You can use the Attach Actor to Component node to attach the Bonus BP to the Platform :
In your Bonus BP, you could do a LineTrace straight down to see if it hits a Moving Platform, and if it hits, you could get the Component that it hit and use the Attach Actor to Component node