Validate a Marketo Terms and Conditions (or any other) Checkbox

checkbox-validate.js

//Add the ID of your Marketo Form

$('#mktForm_1030').attr('onSubmit','return validate_form ( );');

//Add the "name" properties of your form and field. i.e. mktForm_1030, CustomField_Terms_of_Service)

function validate_form ( )
{
	valid = true;

        if ( document.mktForm_1030.CustomField_Terms_of_Service.checked == false )
        {
                alert ( "Please check the Terms & Conditions box." );
                valid = false;
        }

        return valid;
}