call javascript confirm() conditionally and get its ok cancel value ( asp.net)

You can still do

protected void Page_Load(object sender, EventArgs e)
{
if(x == 7)
{
  string message = "Do you want to Submit?";
  System.Text.StringBuilder sb = new System.Text.StringBuilder();
  sb.Append("return confirm('");
  sb.Append(message);
  sb.Append("');");
ClientScript.RegisterOnSubmitStatement(this.GetType(), "alert", sb.ToString());
 }
}