Letterdrop lets you add custom data fields to your subscribers so that you can categorize them and create lists.

Visit letterdrop.com/publications.

Open your publication. Click on the Subscribers button.

Scroll down to Custom Fields and click on Add Custom Field. You can provide the name of the field, the type (number, text, boolean, or date) as well as the default value.

Now, when creating a new subscriber via Zapier or API, you can provide these fields in additionalData.

You will see data for these fields in your subscribers view. You can use these fields to filter your subscribers and create lists.