From AC Help:
I have heard back from our team and want to let you know that ActiveCampaign’s “Create/Update Contact” zap action does not allow you to use double opt-in confirmation.
The only way to use double opt-in confirmation via Zapier is to use Zapier’s own “Code” app as the zap action app.
And then, using that “Code” action, perform an API call to subscribe the contact (using contact_sync) and making sure to pass along the form ID of the ActiveCampaign form that is subscribing people to that list and that has Opt-In Confirmation enabled.
So, I’m off to learn how to do this, but it seems it may be possible… here’s to hoping it’s not too complex!