AJAX :: Capture Onclick Event Of Textbox Then Set Visibility Of Panel.

I need to achieve an effect of a read-only textbox, where when the user clicks on the textbox a popup grid control appears, after the user has made a row selection the text box is populated.... Looking at the AJAX Toolkit model it seems like a CustomExtender would be the way to go, but I can find no documentation on how to build such a thing, so I am looking at improvising.

Here is my process (which is still not working):

Using either an ASP.NET TextBox or HTML <input type="text" runat="server"> control I need to able to trap on the onclick event and then set an ASP.NET Panel control Visible="true". I have tried placing my GridView control inside a <div> and using style properties to set it to hidden and using a client-side (javascript) event handler on <input type="text"> onclick event setting the style property to visible, this does not work correctly. It results in a empty place-holder like grid that popualtes on click (I suspect this is because the DataGridView lives inside a ASP.NET Panel)

I would like to be able to call a server-side method from inside my client-side JavaScript function. I seem to recall from Microsoft AJAX 1.0 docs a way to RegisterClientScript or some approach that allows this and can not find anything on this being supported with the ControlToolkit/MS AJAX 2. If I can do this I could set my ASP.NET Panel Visible property to true and then call Update() on it's UpdatePanel parent which should give me exactly what I'm looking for.