There are two ways you can add a list to your sign-up form:
In the left-hand navigation, click Contacts > Lists > View Lists. Next to the list you would like to use, click the button from No to Yes to add the list to your sign-up form.
Another way to add a list to your form is to go to Forms > Create Sign Up Form and then click Select under Link to Your Sign-Up Form.
Click List Settings on the left-hand side. Using the Show/Hide option, you can decide which lists you want to show up on your sign-up form. Make sure you click Save Changes when you’re finished.
You can also click to hide all of the lists and automatically assign new subscribers into any list by clicking the Auto Assign boxes.