Patrick works as a Software Developer for BlueWell Inc. He develops an ASP.NET application named App1 using Visual Studio .NET. The application displays employee registration form on the company's Web site. The form contains the following fields:

l First Name

l Last Name

l Date of Birth

l E-mail

The application contains a TextBox control for each field. Patrick wants the application to provide a user-defined error message whenever an employee makes a wrong entry on the form. Patrick uses a CustomValidator validation control in the application. Which of the following properties is mandatory if the CustomValidator control is used to validate a user input data entry?