define Custom Event for WebControl in asp.net

Good article which can help you to achieve your task:

Custom Controls in Visual C# .NET

Step 1: Create the event handler in your control as below.

public event SubmitClickedHandler SubmitClicked;

// Add a protected method called OnSubmitClicked().
// You may use this in child classes instead of adding
// event handlers.
protected virtual void OnSubmitClicked()
{
    // If an event has no subscribers registered, it will
    // evaluate to null. The test checks that the value is not
    // null, ensuring that there are subscribers before
    // calling the event itself.
    if (SubmitClicked != null)
    {
        SubmitClicked();  // Notify Subscribers
    }
}

// Handler for Submit Button. Do some validation before
// calling the event.
private void btnSubmit_Click(object sender, System.EventArgs e)
{
    OnSubmitClicked();
}

Step 2 : Utilize the event in the page where you register your control. The following code is going to be part of your page where your control is registered. If you register it, it will be triggered by the submit button of the control.

// Handle the SubmitClicked Event
private void SubmitClicked()
{
    MessageBox.Show(String.Format("Hello, {0}!",
        submitButtonControl.UserName));
}