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.
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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.
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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.
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You can also click to hide all of the lists and automatically assign new subscribers into any list by clicking the Auto Assign boxes.