1. Add Discount on Plan assignment to member
You can select any pre-made discount available for plans at the same time you add the plan to the member through the Control Panel.
2. Add Discount after Plan is assigned to member
Use the modifier/sub-menu on the members detail page on the plan row to add or remove a discount after the plan has been assigned. This will affect future billings if this is a recurring plan.
3. Send prospective member a URL to a plan with discount included
Use the button in the right rail of any plan detail page. 'View Page' found under the heading 'Landing Page' This will pop a modal where you can select which discount to use. It will automatically apply this to anyone that goes to the URL it generates.
You can't 'double stack' a discount. Only one discount permitted per plan.
To send out a plan link with discount, the plan has to be visible publicly.