Hi Jim,
This functionality is built-in to the core calendar and happens before you save the event.
The calendar will automatically set the resource value to the "none" resource (if that's still in your resource list) and
there are no resource filters applied. If you've applied a resource filter, then the resource will be set to the first resource in the list of selected resources. This order is based on the order of the resources in your filter list, from top to bottom.
If you wanted to change this behavior and define a resource to auto-assign based on your own criteria, you could do that as part of an
"On Create" event action.
I hope that helps!
KC