how to tweak form validator.js to only send form if checkbox is checked

I have made a contact form using the example at https://bootstrapious.com/p/how-to-build-a-working-bootstrap-contact-form which works fine. Now I would like to add a checkbox which the user has to check (besides a google recaptcha and the mandatory form fields) if he wants to be able to submit the form.

i think i have to tweak the validator.js file but since i don't know javascript, i have no idea how to edit the existing validator file.

HTML of additional checkbox:

<label>
<input type="chbox" name="chbox" id="chbox"><a href="terms.html">Terms and conditions</a>.
</label>

js:

function getValue($el) {
  return $el.is('[type="checkbox"]') ? $el.prop('checked')                                     :
         $el.is('[type="radio"]')    ? !!$('[name="' + $el.attr('name') + '"]:checked').length :
                                       $el.val()
}

var Validator = function (element, options) {
  this.options    = options
  this.validators = $.extend({}, Validator.VALIDATORS, options.custom)
  this.$element   = $(element)
  this.$btn       = $('button[type="submit"], input[type="submit"]')
                      .filter('[form="' + this.$element.attr('id') + '"]')
                      .add(this.$element.find('input[type="submit"], button[type="submit"]'))

  this.update()

  this.$element.on('input.bs.validator change.bs.validator focusout.bs.validator', $.proxy(this.onInput, this))
  this.$element.on('submit.bs.validator', $.proxy(this.onSubmit, this))
  this.$element.on('reset.bs.validator', $.proxy(this.reset, this))

  this.$element.find('[data-match]').each(function () {
    var $this  = $(this)
    var target = $this.data('match')

    $(target).on('input.bs.validator', function (e) {
      getValue($this) && $this.trigger('input.bs.validator')
    })
  })

  this.$inputs.filter(function () { return getValue($(this)) }).trigger('focusout')

  this.$element.attr('novalidate', true) // disable automatic native validation
  this.toggleSubmit()
}

only if the mandatory fields are filled out and the checkbox and the recaptcha checks are made, the submit button should become clickable and the form can be submitted.