Squarespace has no OptiMonk app or plugin, so you install OptiMonk the universal way: copy one JavaScript snippet from OptiMonk and paste it into Squarespace's Code Injection. It takes a couple of minutes, and afterwards OptiMonk can check the site for you instead of making you wait for traffic.
1. Get your snippet from OptiMonk
Open Settings from your account menu, then the Domains tab.
If your Squarespace domain isn't listed yet, type it into Type a new domain, e.g. example.com and click Add.
Click the domain's row. It is marked Not inserted until the code is live.
OptiMonk takes a look at your site first (Looking at your site…). Squarespace is not one of the platforms with its own step-by-step guide, so you land on the picker: What e-commerce platform do you use?
Choose Custom website.
The guide then shows one thing — Connect your website and your snippet, with a Copy code button.
Other platforms works exactly the same way and gives you the same snippet, so either card is fine.
2. Paste the code into Squarespace
Log in to your Squarespace admin control panel.
Go to Settings → Advanced → Code Injection.
Paste the OptiMonk snippet into the Header field and save.
OptiMonk asks for the snippet in the page head, just before the closing </head> tag, on every page of your site — which is what the Header field does on Squarespace. The script loads asynchronously, so it won't slow the site down, and it only needs to be included once per page.
3. Check that it works
Go back to the guide in OptiMonk and click Check now at the bottom. OptiMonk loads your site once and looks for the code, so you get an answer straight away:
The OptiMonk code is live on {domain}. — you're done.
We looked at {domain} and could not find the OptiMonk code yet. — make sure you saved the Code Injection panel and that the page is publicly reachable (a site still behind a Squarespace password won't work), then Check again.
An OptiMonk code is on {domain}, but it belongs to a different account. — an older OptiMonk snippet is still on the site; replace it with the one you just copied.
After a successful check, the row in Settings → Domains shows Code inserted.
Checking loads your site once and looks for the code — it does not need visitors. If the row later adds no recent traffic, the code is still installed; nobody has simply loaded a page in the last 24 hours.
Can't do this yourself?
In the guide, open Do you need help? and choose Send the instructions to my developer. Enter their name and email address, edit the message if you like, and click Send the instructions — whoever looks after the site gets the snippet and what to do with it, without needing an OptiMonk login.
A note on cart-based targeting
Targeting rules that depend on what is in the visitor's cart are fed by shop data OptiMonk does not receive from Squarespace out of the box. If you want cart rules on Squarespace, that data has to be passed to OptiMonk with a bit of development — see JavaScript code for Cart Rules.