...
3. Fill in all required fields (RED MARK)
Choose Available Key in API Key drop-down (GREEN MARK)
4. If you want to add few fields to your operation Click Add Field = Value pair and fill in the fields (RED MARK), click Save
5. Click Save Operation (RED MARK)
Close Operations window
6. Done. Now you can use this Operation in any trigger or in any Action in mobile your app.
Check one Example - Using API Operation with triggerTrigger an Operation on Filtered data