Set up the birthday email
The birthday email is part of Pro. You set it up once; only clients with a birthday who accept promotions get it. Actual sending is being switched on soon; your message will be ready.
Ask for the birthday
Under Settings, Online booking, set Birthday to "Optional" or "Required".
Open the birthday email
Under Promotions, Birthdays.
Choose discount and message
Percentage or fixed amount, which services, how long it's valid and the code. Adjust the text and switch it on.