A form action link is a URL that’s found within sign-up embed codes. So, for example, if you wanted to configure the Subscriber List widget to work with Mailchimp, you can look for this link by going to Audience > Signup forms. Next, click on “Embedded forms” to generate your sign-up form code.
Where you see form action=, you’re going to copy/paste the link that’s in-between the quotation marks.
These instructions go for any sign-up form code from any newsletter service. Once you have your form action link, simply copy/paste into the Subscribe List settings.