If you are using Kentico 9 for email marketing and want to customize your NewsletterSubscription web part, this guide may help.

I had a requirement to add a required checkbox field to the default subscriber form — for example, a user consent or agreement checkbox that must be ticked before subscribing.

Kentico’s official documentation suggests modifying subscriber fields through the Modules application:
https://docs.kentico.com/k82/on-line-marketing-features/email-marketing/working-with-email-campaigns/managing-email-marketing-subscribers

I made changes in the Newsletter - Subscriber class using the module system table and added a new Boolean field (checkbox).

Here’s what happens:

• The field is added for all current and future subscribers.

• If no default value is set, existing records will have the new field as NULL.

• This will not overwrite or corrupt existing data.

• You should back up your database before making structural changes.

More on editing system tables here:
https://docs.kentico.com/k9/custom-development/editing-system-tables

This approach works well if you want to collect extra information or enforce specific terms for subscriptions.

Let me know if you’ve tried this on newer versions of Kentico or used conditional logic with macro rules for checkbox validation.