Coupon code

If you would like to allow them to post for free using discount codes then this is possible if you are using WooCommerce Integration to charge users for posting Ads.

ok not using woocommerce.

The default Payments Module does not support coupons.

What you could do is use the Pricing Per Capability snippet https://github.com/simpliko/wpadverts-snippets/blob/master/pricing-per-capability/pricing-per-capability.php, then to each pricing, you will be able to assign required capability to see it.

For example using a plugin like Members you can create a capability “free_posting” and assign it to some users, then from wp-admin / Classifieds / Options / Payments / Pricing panel add this capability to selected pricing. Then only users with free_posting capability will be able to post for free.