Wrap the controls in an asp:Panel
and set the DefaultButton
.
<asp:Panel runat="server" DefaultButton="lnkValidateCoupon">
<asp:TextBox ID="txtCoupon" runat="server"></asp:TextBox>
<asp:LinkButton ID="lnkValidateCoupon" runat="server" OnClick="lnkValidateCoupon_Click">Validate Coupon</asp:LinkButton>
</asp:Panel>