Own domain
From SimplyBook.me
If you want to change simplybook.me in url of your booking page, you can enable Custom domain feature in your Custom Features page per 129USD.
Then in your Domain controller, you need to find where you set the DNS records. You should make the A record for your website direct to 158.69.224.50 (51.255.64.80 for simplybook.it companies) as in instructions displayed in this feature settings.
After A-records are set enter domain name you wish to use in this custom feature settings.