Sorry, we don't currently offer a coupon or discount feature.
It's possible to add a "promotion code" field to the booking form (under Setup > Booking Details) so clients can enter a keyword to help you track marketing initiatives, but it doesn't automatically apply a discount.
There are some workarounds you might consider. For example, some businesses create a service named "New Client Special" with a lower price.
Or you might try the Gift Certificates & Packages add-on; it costs $5/mo extra and enables clients to buy and redeem gift certificates and packages online. You could create individual vouchers to apply a discount. For more on Gifts & Packages, please see the Gifts & Packages help section.