When booking an appointment in the Cart you will come across a box that says "Apply Offer". Click on it, you will be shown various offers. On top of the offers section you will find blank field of "Enter Code."
Enter the code you have got in this blank field and click button "Apply" next to it. If you have got a valid promotion/ discount code, on clicking apply button the discount will be automatically applied to your cart value and you will see discounted cart value.