Clickk on MAIL. The Promo Code button is at the bottom of the mail window.

Click on the MAIL button. Promo code button is located at the bottom of that window.

Click the mail button ✉️ on your hangar page as shown below:

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Then in the inbox, click the green button in the bottom right of your screen.

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On iOS, it redirects you to a website to which you enter your ID, then the promo code below.