HTTP requests are simple: method + URL, headers, and request body. EasyMorph doesn’t do any black magic (except calculate Content-Length). It simply sends what you configured. Compare what you send in EasyMorph with what you send in Postman. If it’s the same, there is no reason why the same HTTP request wouldn’t work in EasyMorph.
I’ve googled “xentral” and it seems that the xentral uses different content types. At least there are two options, application/vnd.xentral.default.v1+json and application/vnd.xentral.default.v1-beta+json.
According to xentanl, error http code 415 means that "resource representation send in the request is not supported." You may have chosen the wrong content type or something like this.
Please contact the xental API provider and ask them for the correct Content-Type header value.