How to authenticate XML web api in ASP.NET C#

namespace testwebapi
{
    /// <summary>
    /// Summary description for testwebapi
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class testwebapi: System.Web.Services.WebService
    {
        public class AuthHeader : SoapHeader
        {
            public string Username;
            public string Password;
        }

        public AuthHeader Authentication;


        [SoapHeader("Authentication", Required = true)]
        [WebMethod(Description = "WebMethod authentication testing")]
        public string SensitiveData()
        {

            //Do our authentication
            //this can be via a database or whatever
            if (Authentication.Username == ConfigurationManager.AppSettings["ff_username"] &&
                        Authentication.Password == ConfigurationManager.AppSettings["ff_password"])
            {
                //Do your thing
                string success = "";
                success = "{\"status\": \"test web api security.\"}";
                this.Context.Response.ContentType = "application/json; charset=utf-8";
                this.Context.Response.Write(success);
                return "";

            }
            else
            {
                //if authentication fails
                return null;
            }
        }