If you want to add the checkbox in your form while your subscriber want to subscriber your newsletters. Then you add the check as per your requirement.
For this how to add the checkbox in your form, follow steps below,
Step1:- Login to your The Expo Email account.
Step2:- Click on Email List>>List>>click on existing list.
Step3:- Click on Custom Field.
Step4:- Click on Add Checkbox.
Step5:- Add the checkbox field from here.
Step6:- Once add the check box in form, the form will be like this.
Step7:- Once your subscriber will subscribe or fill the form, that subscriber will be added to your list.