Create a Message of Type Response and channel Email using REST service

Summary:

We have a requirement to create a message on a Service Request on an Event. This message should be of type Response and Channel = Email so that an email is triggered to the Primary Contact on the Service Request. I have tried with the below request payload. The message got created on B2B Service Request as well. But I see an error message -

Content (required):

/crmRestApi/resources/11.13.18.05/serviceRequests/<Service Request Number>/child/messages

{

    "MessageTypeCd": "ORA_SVC_RESPONSE",

    "ChannelTypeCd": "ORA_SVC_EMAIL",

    "SourceCd": "ORA_SVC_AGENT_UI",

    "StatusCd": "ORA_SVC_COMMITTED",

    "MessageContent": "VGVzdGluZyBtZWVhZ2Ugb2YgdHlwZSByZXNwb25zZSBmcm9tIEFQSQ=="

}

Version (include the version you are using, if applicable):