In this doc, we'll show you how to Create Custom Fields in ConvertKit. There are a few additional steps to complete as well -
1. In ConvertKit, click on the 'Subscribers' tab, select any subscriber and then click 'Add a new field' to add your new custom field:
2. Type the name for the new custom field and click 'Update Subscriber':
If you have any questions, please let us know at firstname.lastname@example.org.