The ClickOptin is a link used in an email. So, while the clickfunnel user must have them as a ‘contact’, the contact does not have to already be a clickfunnel contact.
The link will automatically trigger the Submit function from an optin page. The visitor will never see that page but will be instantly directed to the next step or redirect override once the link is clicked in the email. The only contact info added would be an email address. It will not collect or auto prefill a person’s name.
If they are not a known contact in clickfunnels, the ClickOptin will use the link in the email to grab the email (whether it’s coming from Mailchimp, Active Campaign etc) and optin with that information. But it’s not an auto prefill feature from the browser.