Show Alert Message from Code Behind(Server Side)

I want to Show alert in page load

  protected void Page_Load(object sender, EventArgs e)
{
  string message = "Hello! Mudassar.";
  System.Text.StringBuilder sb = new System.Text.StringBuilder();
  sb.Append("");
  ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());
}

Please refer this code.

Html Coding



    
    
    
    Bootstrap 101 Template
    
    
    
    
    
    


    
    
        
                      
            
            
            
            
            
                
                    
                        
                            
                            
                                Registration done Successfully

protected void Page_Load(object sender, EventArgs e)
{
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "ShowPopup();", true);
    this.lblMessage.Text = "Your Registration is done successfully. Our team will contact you shotly";
}