[Solved] Session value set using JavaScript is always NULL inside WebMethod in ASP.Net

When an Input Button is clicked In the aspx it calls a JavaScript that sets up Sessions Variables and calls WebMethod.

The Input Box txtUserID has a value but Session["UserID"] is always null in WebMethod.

Why and how do I fix it?

html:

<div>
    <label for="txtUserID">USER ID:</label>
    <input type="text" id="txtUserID" name="txtUserID" runat="server" style="width:150px;"/>
</div>

Script:

function SetSessionVariables()
{
    //var CurrentDate2 = document.getElementById("txtUserID").value;
    '<%Session["UserID"] = txtUserID.Value; %>';
}

Code Behind (Web Method):

[System.Web.Services.WebMethod(EnableSession = true)]
public static void SaveData()
{
    string sUserID= HttpContext.Current.Session["UserID"].ToString();
}