Adding Custom Headers - HodStudio/XitSoap GitHub Wiki
In case you need to provide some custom header to the webservice, you just need to call the method AddHeader, passing the key and the value for it.
var wsCon = new WebService("http://localhost/XitSoap/ProductService.asmx");
wsCon.AddHeader("CustomHeaderKey", "Custom Header Value");
var result = wsCon.Invoke<Product>("SearchProduct");