I. On offer setting page, scroll down to Landing link eligibility section and click on toggle to turn on:
II. Click "Add keyword" button, input your keyword and click "Save" button. Then also click "Save offer" at the bottom of page.
III. To view offer, you must use a link contains the keyword.
For example: they keyword is ABC, to activate offer, you should enter the link https://your-store-domain?ABC
* Don't forget to turn off this option later to publish the offer.