CiviDiscount for Membership
If CiviDiscount is enabled, you should see the discount box at the top of the membership page even if you don't have any discounts configured.  The position isn't great and can be missed if someone is expecting to see it near the amount.


You don't need to use price sets.

If you're quick, you can see an example at https://dmaster.demo.civicrm.org/civicrm/cividiscount but it is regularly reset.

I noticed though that the discount is not being applied correctly, so I have created a ticket for that.