You can configure your Recurly.js checkout form to add a coupon field to pull the coupon value and apply it to the purchase price. Since the Recurly.js form is completely customized you'll have to build the coupon field and add it to your checkout form.
We have information on configuring the coupon field in our documentation on Github here: https://github.com/recurly/recurly-js-examples/blob/master/public/advanced-pricing/index.html#L52
Utilizing the Github examples and following the rest of the pricing code here: https://dev.recurly.com/docs/recurly-js-pricing the coupon field will be taken into account and show a discounted price based on coupon application. This advanced pricing covers that as well as tax.