private void Button1_Click(object sender, System.EventArgs e)
{
    AuthHeader auth = new AuthHeader();
    WebServices webService = new WebServices();
    auth.UserName = this.txtName.Text.Trim();
    auth.Password = this.txtPwd.Text.Trim();
    webService.authHeader = auth;
    string rtStr = webService.GetPassword();
    this.txtReturn.Text = rtStr;
} 

public class AuthHeader : SoapHeader
{
    public string UserName;
    public string Password;
} 

public class WebServices : System.Web.Services.WebService
{
    public AuthHeader authHeader;
    [SoapHeader("authHeader")]
    [WebMethod(Description = "This method will return the sensitive data")]
    public string GetPassword()
    {
        if (authHeader.UserName.Equals("user") && authHeader.Password.Equals("pwd"))
        {
            return "pwd";
        }
        return "Invalid Authentication ";
    }
} 

System.Net.CookieContainer cookieContainer = new System.Net.CookieContainer();
MyService.WebService service = new App.MyService.WebService();
service.CookieContainer = cookieContainer;

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.web>
    <compilation debug="false"    />
    <authentication mode="Forms">
      <forms name="MyService" loginUrl="Login.asmx"></forms>
    </authentication>
    <authorization >
      <deny users="?"    />
    </authorization>
  </system.web>
</configuration>

public bool Check(string userName, string password)
{
    if (userName == "aaaaaa" && password == "123456")//添加验证逻辑
    {
        System.Web.Security.FormsAuthentication.SetAuthCookie(userName, false);
        return true;
    }
    else
    {
        return false;
    }
}

class Sample
{
    System.Net.CookieContainer cookieContainer = new System.Net.CookieContainer();
    public void Login()
    {
        MyServiceLogin.Login login = new App.MyServiceLogin.Login();
        login.CookieContainer = cookieContainer;
        login.Check("aaaaaa", "123456");
    }
    public void ShowHelloWorld()
    {
        MyService.WebService service = new App.MyService.WebService();
        service.CookieContainer = cookieContainer;
        Console.WriteLine(service.HelloWorld());
    }
}