Donorbox has a feature to flag donors that want to subscribe to your mailing list. This is necessary because one cannot spam donors with unwanted emails. It may even upset them and possibly prevent them from contributing further.
To set up your Donorbox form to get permission to add donors to your mailing, follow these steps:
Log in to your Dashboard and click the Edit link next to your campaign form, as shown.
Select the Email tab of the form builder, as shown.
You will see a Toggle button, labelled "Ask donor to subscribe to our mailing list". Click it to turn this feature on, as shown.
You can customize the label to whatever you'd like it to display. You may also choose to have this option already selected by default in your donation form.
The form will then display this option, as shown.