Instead of using an actual form button to open the modalpopup, assign the targetcontrolid to a hidden(use style="display:none;", not visible=false as javascript can't see it then) asp:button. Then you can control it from code using .Show() and .Hide()

 Eric

There's a ModalPopup.Show() method which can be used on the server-side. 

Can you please  help me with some link to help me understand how to do this please???

If you need help with my suggestion, you just need to call the Show() method from your button click event, e.g.

protected void Button1_Click(object sender, EventArgs e)
{
    MyModalPopup.Show();
}

<asp:Button id="btnButton" runat="server" style="display:none" />

ID="mpe" runat="server" PopupControlID="myPanel" TargetControlID="btnButton" DropShadow="false"  />

 In your code behind you can call mpe.Show() to display the modal popup and have another function call mpe.Hide() when you want it to disappear.

Tim

BehaviorID="programmaticModalPopupBehavior"

TargetControlID="BtnAddTransaction"

ID="ProgrammaticModalPopUp"

PopupControlID="WSMessenger"

runat="server"

BackgroundCssClass="modalBackground"

<div id="WSMessenger" runat="server" style="display: none;width: 300px">

<div style="float: right;">

<asp:LinkButton ID="DivClose" runat="server">Close</asp:LinkButton>

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;

</div>

<div id="WSMessageArea" runat="server">

</div>

Me.ProgrammaticModalPopUp.TargetControlID = "BtnAddTransaction"

End If

End Sub

You need the TargetControlID to be something other than BtnAddTransaction. Create a new button as suggested and set it's style to "display:none" or use a Hidden Field instead. Then you won't get the popup displayed when you click on BtnAddTransaction and you can call the Show() method on the server to show it.