By setting the value of your input that has name="cb" to something sensible. For example to 1. Then, when it’s checked, it will send that as part of the form, and when unchecked, it won’t send anything (its name won’t be present in the form data unless there are multiple elements with the same name and at least one of them is checked).

You absolutely don’t even need JavaScript for this.

As for catching the form data in Mason — no idea, I’ve never used Mason. Its documentation surely talks about how to deal with data sent via forms.