Not sure I am following the correct format for the fields defined in the app. I have added the HTTP Alert Action to my alert and entered the required values but I am unable to confirm if the field format is correct?
I personally use the excellent RequestBin.com site to test all my config.
Should be able to identify errors with the following search. "index=_internal sourcetype=splunkd component=sendmodalert action="send_custom_rest_request"