#1 - People can manually enter the coupon on the Checkout Page. For this you don't need to do anything else other than providing the coupon code to them.

#2 - You as the product seller can automatically apply the coupon using a link.

Simply attach the following string AT THE END of the ORDER URL:
?c=coupon-code

where coupon-code is the actual coupon code.

E.g. You have this coupon code:
birthday-special

Scenario 1
----------
Suppose your ORDER URL is:
https://username.zaxaa.com/o/1234567/1

Then the coupon link would be:
https://username.zaxaa.com/o/1234567/1?c=birthday-special


Scenario 2
----------
Suppose your ORDER URL is:
https://username.zaxaa.com/o/1234567/2

Then the coupon link would be:
https://username.zaxaa.com/o/1234567/2?c=birthday-special

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