When you add or edit an email template, it has a "Enter coupon code to add into email" Here you need to 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 to all customers to whom that abandon cart email is sent to.

If you want to send a unique coupon code to each customer, then you need to enable the "Generate unique coupon codes" setting. In addition, you still need to enter a parent coupon code in the "Enter a coupon code to add into email" setting. The unique coupon codes will be generated based on the settings added for the parent coupon code.

You can learn more about the coupon code settings here.