For V2 checkout pages:
If you want to pass a discount code to the checkout page URL, just make use of the following syntax:
your-domain.chargebee.com/hosted_pages/plans/plan_id?subscription&subscription[coupon]=coupon_name
Here's a sample URL for reference:
https://zeon-test.chargebee.com/hosted_pages/plans/cbdemo_bonanza?subscription&subscription[coupon]=CBDEMO_SAVER
For V3 checkout pages:
You can include a coupon code to the checkout script so that it is automatically applied. It can be done similar to the example mentioned in the "Add-on customizations" section in this document. The script code to include coupons is as mentioned below.
var cbInstance = Chargebee.getInstance();
var link = document.querySelectorAll(“[data-cb-type=checkout]“)[0];
var product = cbInstance.getProduct(link);
product.addCoupon(“COUPON NAME”);