The Create Incident API accepts only Description, OpenDateTime, DueDateTime, and ClientId fields while creating an incident. To populate other fields such as template or custom fields for an incident, I suggest using the Salesforce platform REST API directly. See https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_sobject_create.htm
Thanks for the quick reply.
I thought this might be the answer but thank you for setting the record straight for me.