When you add or edit an email template, it has a setting "Enter coupon code to add into email" where you can enter a coupon code that you have created from WooCommerce > Coupons page. You would also need to add the {{coupon.code}} merge tag in the Body section of the template. This will send the same coupon code in the reminder emails.

If you want to send a unique coupon code to each customer, then you need to enable the "Generate unique coupon codes" setting. You can set up the corresponding settings for the unique coupon code there. You can learn more about the coupon code settings here.