showModalDialog[
^] is in the process of being dropped, and should not be used. It was removed from Chrome in v37 back in 2014, and Firefox will drop support in June this year.
You will need to change your code to use an alternative - for example:
The modal dialog content will then be part of your main page. You should be able to add a
<asp:CheckBox AutoPostBack="True">
to the content, which will trigger a post-back when the user checks the box.