A Google Ads promo code for the desired amount.

Spend money on advertising within 🕗 60 days. Once you've spent the full amount, you'll receive a $1 bonus 🎁 shortly afterward. The bonus may take up to 35 days to be credited, but it's usually credited more quickly. This applies to all coupons and all countries.

For the bonus to be added to your account, your Google Ads account spend must reach the stated minimum (excluding taxes and fees) within 60 days of entering the promo code.
You can see the credited amount in the "Billing Summary" section.