Expose your modal popup as a public property on your main page.
public class BasePage: System.Web.Ui.Page
{
public ModalPopupExtender MyPopup
{
get
{
return this.myPopup;
}
}
}
In your ascx code behind cast this.Page
to your page type.
public class Page1 : System.Web.UI.UserControl
{
protected void Page_Load(object sender, EventArgs e)
{
((BasePage)this.Page).MyPopup.DoWhatEver();
}
}