Form designer: How to add custom code on the submitButton event in the ASP.NET server side code-behind?

// PageLoad 
protected void Page_Load(object sender, EventArgs e) 
    base.Page_Load(sender, e); 
    submitButton.Click += new EventHandler(MySubmitButton_Click); 

protected void MySubmitButton_Click(object sender, EventArgs e) 
    // Your code... 
    // If you modify the form field's value then you have to call the 2 instructions below at the end.
    // this.SaveFormData(this.FormData, true); 
    // SubmitToWorkflow(); 

Note: This custom event will happen after the normal WorkflowPage submit event.