Under Your shop, Sell more, Offers, add an offer: a headline ("10% off for Christmas") and, if you like, a line under it ("Use the code when you book. Ends Christmas Eve."). It sits across the top of your booking page, where people choose what to book, in your page's colours.
Pick one of your discount codes and the offer shows it with a Use code button: tapping it copies the code and fills it in for the booking, so it comes off on the last step without typing. Make the code first under Sell more, Discount codes; any code people type can go in an offer, and the code's own rules (who can use it, how often, a minimum spend) still apply. With no code, the offer is just a message: news, a new service, the days you are closed.
Choose the services that wear the offer's badge ("10% off", or "Offer" with no code), or every service. Give it a Shows from and an Until date and it comes and goes by itself, with "Ends tomorrow" on the last days. Switch it off any time; an offer whose code you retire, or whose code has run out, comes off the page by itself.
Each offer has a share link: your booking page with the code riding along. Post it on Facebook or Instagram and whoever taps it has the code filled in when they book. Up to three offers show at once, the newest first. Offers are on Pro, like discount codes, on the website and in the FullDiary app.