How can I set checkbox checked outside modalbox via jQuery

I have a form on that form I have some textbox, buttons, radio button and one checkbox.

On one button's click the modal box will appear on screen and model will contain 2 button and 3 input.

When model open the overlay will grey out the background.

When we click one of the button on model I need to set checkbox checked on form behind modal overlay but unable to set it via jQuery.