The Form Action Code is what connects this widget to your Mailchimp list.
To get this code, start by creating a list. Once your list is created, open it and find the Sign up forms menu option. Then click Embedded forms.
Inside Embedded forms, open the Naked menu option. Scroll down and you'll see a bunch of code. Near the beginning of the code, you'll see an
action attribute. Copy everything inside the quotes of that attribute.
Now, go into Muse, open the Form Information section of the widget and paste the copied code into the Form action code textarea.