Go here [1]. Click End membership. Select the option to end later. Note the day your membership ends.

Then go here [2] and buy for yourself as a gift for the day after your membership ends.

[1] https://www.amazon.com/gp/primecentral/editMembership/ref=pc...

[2] https://www.amazon.com/gp/prime/pipeline/prime_gifting_landi...

The discount is for new members only.

If you read the article in it's entirety you'll see it specifically mentions current customers are not eligible for the promotion.