Hi Ashok, you can name the button itself Aprove and Delegate in the Tasks Form buttons.

In regard to your process, without having to resort to Javascript, I would just create a Checkbox on the Form that would say Delegate, and if they click it, then use a field rule to displayed the required Email field. I would only have the Approve button at that point. You can then use the Delegate Checkbox as a condition in your process design to push to email the Delegated user or assign them a task. FYI, typically LF wants the username not the email address if you are assigning a task to a user.