To embed your Online Bookings mini-site into Squarespace, you will first need to go to the Online menu option in your Simple Salon account. Then go to the Admin tab.
Expand General Settings and under the BOOKING SITE URL & CODE section you'll see your code in the Embed Code field. Click Copy Embed Code to copy it to your clipboard.
Once you have your embed code, you will need to follow the instructions as outlined by Squarespace here.
If you have further questions in regards to embedding the code into Squarespace, please contact their support team here.