Hi Partner,
Form field will be added to marketing form and it is mapped to a specific field of Contact or Lead.
In Dynamics 365 for Marketing, the form field record "Do not email" has been mapped to "Do not allow Bulk Emails" field of Contact.

It will be rendered as a checkbox element, and the "Two option" field type indicates that the form field is mapped to a boolean type field.
https://docs.microsoft.com/en-us/dynamics365/marketing/marketing-fields#field-type-and-format-options
In a word, form field is different from field of entity.
In customization menu of Contact entity, we can see the actual mapped field.

The mapped field has been also added to Contact form > Details tab > CONTACT PREFERENCES section.


We just need to drag the corresponding form field to custom subscription center form.
Result

If we submit the form with "Do not email" checked, Bulk Email field of contact will be set to "Do Not Allow".

Regards,
Clofly