Put your form inside an asp.net panel control and set its defaultButton attribute with your button Id. See the code below:
<asp:Panel ID="Panel1" runat="server" DefaultButton="Button1">
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Send" />
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>
Hope this will help you…