how to set a default 'enter' on a certain button

The easiest way is to put the fields and button inside of a Panel and set the default button to the button you want to be activated on enter.

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

if you need to do it from code, use

Me.Form.DefaultButton = Me.btn.UniqueID

Where btn is your button control.

You can use the DefaultButton property on either a server-side form control or Panel control. In your case, group the controls together in a Panel that should fire off the same button:

<asp:Panel ID="SearchBox" runat="server" DefaultButton="BtnSearch">
    ...
    <asp:Button ID="BtnSearch" runat="server" Text="Search!" />
</asp:Panel>
....
<asp:Panel ID="UserPanel" runat="server" DefaultButton="BtnUserSubmit">
    ...
    <asp:Button ID="BtnUserSubmit" runat="server" Text="Submit" />
</asp:Panel>

You can now use UseSubmitBehavior property to disable all the buttons you don't want to fire when hitting submit (check out the documentation for more info)

    <asp:Button ID="BtnNotToFIre" runat="server" Text="Search" UseSubmitBehavior="false" />

how to set a default 'enter' on a certain button

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