This problem should be solved using a combination of a controller and js code. 

1: Create a mail template (if needed)

2: Create a controller endpoint. In this endpoint you want to retrieve the checkbox value and trigger the email template you created earlier.
(docs: https://www.odoo.com/documentation/14.0/developer/reference/addons/http.html)

3: Add JavaScript to your checkbox/form to send the value to the endpoint you have created
(docs: https://www.odoo.com/documentation/14.0/developer/reference/javascript/javascript_reference.html)

For anything more specific it would be required to see actual code