NetCore 3.1 HttpResponseMessage Post([FromBody] return custom XML response - api

Question

I'm new to creating API's and I'm using Visual Studio 2019. I'm trying to write a simple API that our customer passes XML to us, I save the XML to a file for later processing and return back to the customer a custom XML response that they require. I created a .NetCore 3.1 Microsoft.AspNetCore.Mvc API. The code is simple:
[HttpPost]
[Route("api/Pass_XML_to_BOOMI")]
[Consumes("application/xml")]
public HttpResponseMessage Post([FromBody] XmlDocument IncomingXML)
{
XmlDocument incomingXML = new XmlDocument();
XmlDocument outgoingXML = new XmlDocument();
String payLoadID = Guid.NewGuid().ToString();
String curDate = System.DateTime.Now.ToString();
string INBOUND = IncomingXML.OuterXml.ToString();
string xmlresponse = #"<?xml version=""1.0"" encoding=""UTF-8""?><!DOCTYPE cXML SYSTEM ""http://xml.cXML.org/schemas/cXML/1.1.009/cXML.dtd"">
<cXML payloadID=""" + payLoadID + #""" xml:lang=""en-US"" timestamp=""" + curDate + #"""><Response>
<Status code=""200"" text=""OK""/></Response></cXML>";
outgoingXML.LoadXml(xmlresponse);
//grab order ID number and save to file
string ID = "";
XmlElement element = IncomingXML.SelectSingleNode("cXML/Request/OrderRequest/OrderRequestHeader") as XmlElement;
if (element != null)
{
ID = element.GetAttribute("orderID");
}
string filename = #"f:\" + ID + ".xml";
System.IO.File.WriteAllText(filename, INBOUND);
// Create the response
var response_back = new HttpResponseMessage(HttpStatusCode.OK)
{
Content = new StringContent(outgoingXML.InnerXml, Encoding.UTF8),
};
return response_back;
}
The problem I'm having is when I pass the XML in through Postman all works fine and my file is created but no matter what I try the response back is missing my custom XML (it seems be completely ignoring my Content =) and it is in JSON format, not XML.
Here is how the response comes back in Postman:
{
"version": {
"major": 1,
"minor": 1,
"build": -1,
"revision": -1,
"majorRevision": -1,
"minorRevision": -1
},
"content": {
"headers": [
{
"key": "Content-Type",
"value": [
"text/plain; charset=utf-8"
]
}
]
},
"statusCode": 200,
"reasonPhrase": "OK",
"headers": [],
"trailingHeaders": [],
"requestMessage": null,
"isSuccessStatusCode": true
}
I need to somehow send back an XML response with my custom XML:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE cXML SYSTEM "http://xml.cXML.org/schemas/cXML/1.1.009/cXML.dtd">
<cXML payloadID="83dbfb00-22cc-4cb9-9e6b-f34b47f3cbcd" xml:lang="en-US" timestamp="4/2/2022 12:18:49"><Response>
<Status code="200" text="OK"/></Response></cXML>
Does anyone have any idea what I'm doing wrong and how I can accomplish this? Thank you all in advance.