[WebMethod]
public string TestMethod(string a)
{
    string a = DB.Test(a);
    return a;
}

 HttpWebRequest httpWebRequest = (HttpWebRequest)
                    HttpWebRequest.Create("http://www.******.ru/*********/*****.jsp");
                    httpWebRequest.Method = "POST";
                    httpWebRequest.ContentType = "text/xml";

<xml>
<function>Test</function>
<params>
<code>001</code>
<name>User</name>
</params>
</xml>

HttpWebRequest httpWebRequest = (HttpWebRequest)
                    HttpWebRequest.Create("http://www.******.ru/*********/*****.asmx");
                    httpWebRequest.Method = "POST";
                    httpWebRequest.ContentType = "text/xml";

<xml>
<result>0</result>
<error>OK</error>
</xml>