ModalPopupExtender OnShow Event

When clicking a button, I have a ModalPopupExtender (Panel) that gets displayed. I can't seem to trigger any event during that process.

Adding an event to the button that opens the ModalPopupExtender doesnt work, as it never triggers. Adding the button as a trigger in my UpdatePanel also doesnt work.

Scenario: I have a gridview with a button outside of the gridview. Clicking the button opens up a ModalPopupExtender which needs to display the checkboxes clicked on the gridview. I have the code to get the checkboxes clicked on the Gridview, however I cant seem to use an event during the process of opening the ModalPopupE开发者_Go百科xtender.