The checked attribute belongs to input fields of type="checked".
If you examine the page's HTML, you'll see that when you add a CSS class to a form field, it's added to its container:

That's why your code doesn't work. The td element on which the class is located doesn't have a checked attribute.
The input element is nested inside the container, so you need to select that. Each option in a checkbox field is its own input field.
The code below will show a popup when one of the checkboxes in a checkbox field becomes checked:
1 | $( ".mycheckbox input" ).on( "change" , function () { |
3 | alert( "i am checked!" ); |
If your checkbox field has multiple options and you want to trigger the event only for one of them, you can narrow down your selection by adding an attribute selector:
1 | $( ".mycheckbox input[value=choice_1]" ).on( "change" , function () { |
3 | alert( "i am checked!" ); |
That will attach the event handler only to the checkbox option with value "choice 1".