How to get wsdl input and output names to appear

Question

I have a WCF web service project (VB.NET 3.5), and the WSDL it generates does not have the input and output names, like this:

<wsdl:operation name="getListing">
  <soap:operation soapAction="getListing" style="document" /> 
  <wsdl:input>
    <soap:body use="literal" /> 
  </wsdl:input>
  <wsdl:output>
    <soap:body use="literal" /> 
  </wsdl:output>
</wsdl:operation>

I need the input and output names to be defined in the WSDL, like this:

<wsdl:operation name="getListing">
    <wsdlsoap:operation soapAction="getListing"/>
    <wsdl:input name="getListingRequest">
        <wsdlsoap:body use="literal"/>
    </wsdl:input>
    <wsdl:output name="getListingResponse">
        <wsdlsoap:body use="literal"/>
    </wsdl:output>
</wsdl:operation>

This is the definition for the method in question:

<OperationContract(Action:="getListing")> _
    <WebMethod(Description:="Retrieve Base64 binary.", EnableSession:=True)> _
    Public Overloads Overrides Function getListing(<System.Xml.Serialization.XmlElementAttribute([Namespace]:="http://namespacehere.com")> ByVal getListingRequest As ListingRequest) As ListingResponse

I feel like I'm missing something stupid, does anyone know what it is? There are so many options and parameters, I can't find the right ones that determine this.