<asp:TextBox ID="TextBox1" Runat="server">My Initial Value</asp:TextBox>
 &nbsp; 
<asp:RequiredFieldValidator ID="RequiredFieldValidator1"  
 Runat="server" Text="Please change the value of the textbox!" 
 ControlToValidate="TextBox1" InitialValue="My Initial Value">
</asp:RequiredFieldValidator>
