Web Forms :: Set The Panel.defaultbutton To A Button In A Custom Control?

On my page i have a set of control containing parts of a form, like textboxes, checkboxes etc. I use those parts to build controls that I use in pages. For example:

UserAddToList.ascx
<asp:Panel ID="Panel1" runat="server" >
<sikt:PartFormTextboxLine runat="server" ID="samAccountName" />
<sikt:PartFormTextBoxLine runat="server" ID="name" />
<sikt:PartFormButtonLine runat="server" ID="addButton" UseSubmitBehavior="true" />
</asp:Panel>

PartFormButtonLine ascx:

<div>
<asp:Button ID="Button" runat="server" onclick="Button_Click" />
</div>

What I then want to do is to set the behavior of Panel1 to have a default button in the addbutton control. I have tried setting the Panel 1 default button to the botton in formbutonline like this: Panel1.DefaultButton = addButton.Button.UniqueID; in Page_load, but that does not work because .net throws this error: The DefaultButton of 'Panel1' must be the ID of a control of type IButtonControl. Now the button is public, so that should be ok. I have also tried ID and ClientID, with te same results. I have also tried to implement the IbuttonControl in PartFormButtonLine and map the implemented functions to the button

functions, which, as expected did not work. (Panel1.DefaultButton = addButton.ID) The problem as far as I can tell is that I cant seem to set the

"onkeypress="javascript:return WebForm_FireDefaultButton(event, 'MainContent_UserAddToList_addButton')"

of the container div to the correct value, onkeypress="javascript:return WebForm_FireDefaultButton(event, 'MainContent_UserAddToList_addButton_Button')" Everything else works, this is the last thing that needs to be fixed.. Idealy i want to fix this in the PartFormButtonLine file, as that would be better for reuse {update: removed quite abit of clutter in the ascx snippets}