I hope you are seeking solution for a progress Indicator before your pop up dialog, to show the progress of Loading your Popup ,if yes then here is how you can implement this Progress Indicator along with Modal.
Step 1.
I have taken a simple Panel ,Inserted an Image in it and "Loading Please wait..." is the text I am trying to display.I have given set the Position of the panel to my desired Location on screen
<asp:Panel ID="ProgressIndicatorPanel" runat="server" Style="display: none" CssClass="modalPopup">
<div id="ProgressDiv" class="progressStyle">
<ul class="ProgressStyleTable" style="list-style:none;height:60px">
<li style="position:static;float:left;margin-top:0.5em;margin-left:0.5em">
<asp:Image ID="ProgressImage" runat="server" SkinID="ProgressImage" />
</li>
<li style="position:static;float:left;margin-top:0.5em;margin-left:0.5em;margin-right:0.5em">
<span id="ProgressTextTableCell"> Loading, please wait... </span>
</li>
</ul>
</div>
</asp:Panel>
Step 2
As i want to disable the rest of the screen functnality during the Progress Indicator display I have used a modal Dialog as shown below.
<asp:ModalPopupExtender ID="Progress_ModalPopupExtender" runat="server" ClientIDMode="Static" BehaviorID="ProgressModalPopupBehaviour"
PopupControlID="ProgressIndicatorPanel" TargetControlID="DummyDialogButton" BackgroundCssClass="ModalPopupBG"
RepositionMode="RepositionOnWindowResizeAndScroll" Drag="false" DropShadow="true">
</asp:ModalPopupExtender>
Step 3
Now you need to write Javascript function by including this
$("#ProgressImage").show();