Adding opt-in marketing to Squarespace form

Flow:

Someone completes form on website, owner checks application and either accepts or denies, individual goes through to workflow to receive automated email to create an account and book a market stall once account created.

Issue:

Adding marketing opt-in to squarespace form? Why isn't this possible? Is there a way to add code to do so?

I've been able to look at tagging accounts with 'accepted' 'declined' and hopefully will be able to set up automation from that, but they still need to have the opt in to receive marketing comms.