In CoreWCF, how can I replace references to http://tempuri.org/ with my own namespace?

I am trying to make a CoreWCF SOAP endpoint which is backwards compatible with a legacy .NET Framework SOAP API. One problem remaining is that if I call any method (OperationContract), the XML output references http://tempuri.org/ instead of my custom namespace. I can't seem to see any way to change this value. Here's a snippet of my code:

[ServiceContract(Name = "UploaderAPI")]
[XmlSerializerFormat]
[XmlRoot(Namespace = "http://x-rm.com")]
public interface IUploaderAPISoap
{
    [OperationContract(Action = "http://x-rm.com/uploaderapi/HelloWorld")]
    HelloWorldResponse HelloWorld(string echo);
}

But that gives this output in Postman: Please see line 3 which mentions xmlns="http://tempuri.org/

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
    <s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
        <HelloWorldResponse xmlns="http://tempuri.org/">
            <HelloWorldResult>
                <HelloWorldResult xmlns="http://mileagecount.live.x-rm.com/uploaderapi">Hi there, foo</HelloWorldResult>
            </HelloWorldResult>
        </HelloWorldResponse>
    </s:Body>
</s:Envelope>

I believe this is what's causing the client application to reject the response.