Authentication token issue in web service call (.asmx)

Hi All

I have third party web service (.asmx) url , which i am going to consume in simple asp.net web application

i have added service reference of this service and calling its method but it is asking parameter of type tokenheader for each and every method.

i have credential too to access this service , but my question is how to pass first parameter tokenheader

VTrack.PositioningWSSoapClient proxy=new VTrack.PositioningWSSoapClient();
proxy.GetGPSPositionsSinceDate(tokenheader,Datetime);

if i add web reference of this service it ask only one parameter 

proxy.GetGPSPositionsSinceDate(Datetime);

but it gives error 

System.Security.Authentication.AuthenticationException: Invalid Token. Please log in. OR    Invalid Token. authentication token must be set.

Please help i stuck here,

Thank you