Forms Authentication, Web Service, and AJAX

Hello,

I'm using a plain vanilla AJAX (not the ASP.NET Ajax) in my web application:

// Construct the SOAP message.
// The name of the Web Service method: GetData
// Parameters: CatID, pageSize
var soap = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
    "<soap:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"
xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"
xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\">" +
    "  <soap:Body>" +
    "    <GetData xmlns=\"http://www.mywebsite.com/webservices\">" ...