Hi,
I contacted Squarespace customer service and received this response:
There's not a built in early access setup for codes or products, but a workaround could be to create a unique early access Store page in the site's Not Linked section which is password protected. That way, only users with the password can access it, and we have more on Not Linked here:
https://support.squarespace.com/hc/en-us/articles/360025899552
And page passwords here:
https://support.squarespace.com/hc/en-us/articles/205814618
You can add early access users to an Email Campaign list with the password, so only those users get it. Once ready, you can then move the products to the site's general access Store page with these steps:
https://support.squarespace.com/hc/en-us/articles/205811428
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Hope that helps! Seems like you can only do it with password at this time.