On the checkout page, tap Have a code? under the price, type your code and tap Apply. A green tag shows what it took off before you pay.

Steps

Tap any upgrade button in Aim ("Get Aim Pro", the Pro lock screen, a Pro pop-up). It opens the checkout page. No account yet? Aim asks you to sign up first, then brings you straight back.
Pick Yearly or Monthly at the top of the price card. Do this before you add your code. Switching plans starts a fresh checkout and clears the code.
Wait for the payment form to load. Then tap Have a code? under the price.
Type your code in the Promo code box. It turns into capital letters as you type, so don't worry about that. Tap Apply. The button says "Checking…" for a second.
It worked if you see a green tag like "CODE applied: $10.00 off", and Aim does a little happy bounce. The total and the "Then …" line underneath update to match.
Pay as usual. The button says Start my free week on yearly and Subscribe now on monthly. See How do I upgrade to Pro?.

Changed your mind? Tap the small x on the green tag to take the code off.

Why didn't my code work?

A red line under the box tells you why, straight from Stripe ("That code didn't work." if it doesn't say more). The usual reasons:

A typo. Check for a zero where an O should be.
It ran out. Codes can expire, or only work a set number of times.
It's for the other plan. Some codes only work on yearly or only on monthly. Flip the switch and try again.
You already used it. Some codes only work once per person.

Still stuck? Email support@adeimmersed.com with the code, or poke me on Discord.

Only at checkout. There's no place to add a code after you've subscribed. Forgot yours? Email me and I'll sort it.
Not on the upgrade pop-up. The pop-up used to have an "Enter Promo Code" link that showed half prices checkout never charged. It's gone: every code goes in on the checkout page, where Stripe checks it.
One code at a time. To try another, take the first one off with the x.
If our checkout page can't load, Aim sends you to Stripe's own payment page ("Taking you to our secure payment page"). Codes work there too: look for Stripe's promotion code link.
The free week still counts. On yearly, a code changes what you pay after the free week. The card tells you the amount and the date.
Same on phone and desktop.