Sets whether a standard field must have a value, and the value used when it has none. The change applies to the whole account. No permission beyond API access is checked, so any API user can change these settings. PATCH is accepted and does the same.

• isRequired must be sent.
• Sending isRequired true needs a non-empty defaultValue, except for contact.email, even when the field is already required. "0" counts as empty.
• contact.email cannot be made optional.
• When the field is contact.first_name, contact.last_name, contact.email or contact.phone, at least one contact standard field must stay required.
• A missing or null defaultValue leaves the stored default as it is. Any other value replaces it, so an empty string clears it.

Each broken rule returns 400 and changes nothing. 404 if the account has no setting with this id. Custom fields are changed elsewhere: contact custom fields with PUT /fields/{id}, and deal and account custom fields with /dealCustomFieldMeta and /accountCustomFieldMeta.