<asp:Panel ID="p" runat="server" DefaultButton="myButton">
  <%-- Text boxes here --%>
  <asp:Button ID="myButton" runat="server" />
</asp:Panel>

<asp:TextBox ID="TextBox1" runat="server" onkeypress="return EnterEvent(event)"></asp:TextBox>
<asp:Button ID="Button1" runat="server" style="display:none" Text="Button" />

function EnterEvent(e) {
        if (e.keyCode == 13) {
            __doPostBack('<%=Button1.UniqueId%>', "");
        }
    }