pop up window from gridview header template

Question

Hi experts,
i need to create a pop up window from the gridview header template, indicated below with this - (?)
but i can't seem to place the header's ID into the page_load, how should I do to let it pop up from the header's template? 

Below is the code in aspx.cs :
protected void Page_Load(object sender, EventArgs e)
{
OpenPopUp(HelpLink, "QualityHelpRate.aspx", "Help", 450, 650);
}
public static void OpenPopUp(System.Web.UI.WebControls.WebControl opener, string PagePath)
{
string clientScript;
//***Building the client script- window.open***//
clientScript = "window.open(\'" + PagePath + "\')";
//***regiter the script to the clientside click event of the 'opener' control**//
opener.Attributes.Add("onClick", clientScript);
}
public static void OpenPopUp(System.Web.UI.WebControls.WebControl opener, string PagePath, string windowName, int width, int height)
{
string clientScript;
string windowAttribs;
//Building Client side window attributes with width and height.//
//Also the the window will be positioned to the middle of the screen//
windowAttribs = "width=" + width + "px," + "height=" + height + "px," + "left=\'+((screen.width -" + width + ") / 2)+\'," + "top=\'+ (screen.height - " + height + ")/ 2+\'";

//***Building the client script- window.open, with additional parameters***///
clientScript = "window.open(\'" + PagePath + "\',\'" + windowName + "\',\'" + windowAttribs + "\');return false;";
//regiter the script to the clientside click event of the 'opener' control*****///
opener.Attributes.Add("onClick", clientScript);
}