How can I change the content type on XmlServiceClient.
var xmlClient = new XmlServiceClient(url);
I need to change from "application/xml" to "text/xml.
It's embedded in the XmlServiceClient so you'd need to override it:
class XmlTextServiceClient : XmlServiceClient
public override string ContentType => "text/xml";
But since you're likely talking to a 3rd Party API you may want to consider using the XML APIs in HTTP Util extension methods which can be used as a general purpose HTTP client that isn't as opinionated towards ServiceStack.