TabPanel关闭按钮

I have an asp.net website in which I'm using an AJAX TabContainer. Within this container I have one TabPanel. Inside this panel I want to have a close button which would, upon clicking, close (visible=false) this panel.

Sample:

<cc1:TabContainer ID="tbcMain" runat="server" Width="100%" ActiveTabIndex="0" CssClass="Tab">
  <cc1:TabPanel ID="tbp1" runat="server" Visible="false">
  <HeaderTemplate>
    <asp:Label ID="lbl_tbp1_Title" runat="server"></asp:Label>
    <asp:ImageButton ID="imb_tbp1_Close" runat="server" ImageUrl="~/Images/cross.png" />
  </HeaderTemplate>
  <ContentTemplate>
    some content
  </ContentTemplate>
</cc1:TabPanel>
</cc1:TabContainer>

The button imb_tbp1_Close does not cause a postback. I belive it's something with the whole region of HeaderTamplate being clickable and thus my button stays behind (like a z-index of sorts).

How can I make my button receive the click and cause a postback?

Edit, the solution I'm using now:

Firstly I could use an actual button event in the code-behind but ImageButton control does not have the property UseSubmitBehavior to set it false. So I handle the event in my Page_Load.

<cc1:TabPanel ID="tbpCust_1" runat="server" Visible="false">
  <HeaderTemplate>
    <asp:Label ID="lbl_UC_CUSTO_1_Title" runat="server"></asp:Label>
    <asp:ImageButton ID="imb_CUSTO_Close_1" runat="server" ImageUrl="~/Images/cross.png" OnClientClick="javascript:__doPostBack('imb_CUSTO_Close_1', '0')" />
  </HeaderTemplate>
  <ContentTemplate>
    <ucCUSTO:UC_CUSTO ID="UC_CUSTO_1" runat="server" />
  </ContentTemplate>
</cc1:TabPanel>

and in the code-behind:

 Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

        Dim evtTarg As String = Request.Form("__EVENTTARGET")
        Dim evtArg As String = Request.Form("__EVENTARGUMENT")

        If evtTarg = "imb_CUSTO_Close_1" Then
            UC_CUSTO_1.clear_fields()
            tbcStranke.Tabs(0).Visible = False

        ElseIf evtTarg = "imb_CUSTO_Close_2" Then
            UC_CUSTO_2.clear_fields()
            tbcStranke.Tabs(1).Visible = False

        End If

End Sub