JSON format via Carrier notification

We are currently trying to migrate our On prem system 6.5.3 to OTM Saas. In one of our solution we are connecting an external System to a carrier by updating the External System ID in the Notification tab of the Service provider screen. And we send out Tender offer to the carrier by doing secure resources after selecting this Carrier on the Shipment . We are wanting to send out the Tenderoffer in the JSON Format but when we try secure resourcing OTM is sending an XML. We have made the External System Content type as application/json and