I've tried to use messagecontract but I get message error if i use:

<OperationContract()>
Function obtenerDatosCliente(ByVal num As String) As Asegurado

If you want to use this operation contract I have to define:

<OperationContract()>
Function obtenerDatosCliente(ByVal num As InputRequest) As Asegurado

<MessageContract()>_
public class InputRequest 

<MessageBodyMember()>_
Public num as String 

End Class 

Can not work with simple data entry using MessageContract??