In order, to add more fields and/or checkboxes to the subscription form you need to create “custom fields” in the subscriber list. Once the custom fields are created – they will appear in the subscription form’s “fields” tab.
Here is the step by step instructions on the matter:
Log into your account and select “Subscribers” section on the left-hand side. Then, click on a button “Custom fields” on the right-hand side and you will be re-directed to a page with a list of all custom fields throughout your account. Click on the “Create custom field” button to create a new custom field. After that is done you’ll be able to select the field to be shown in your form.
There’re several custom field types:
When you are finished with customizing a new custom field – go to “Forms” section, set-up the subscription form or edit the existing one – select “Fields” tab and you will be able to see the custom field you’ve just created.