“Custom” fields are fields that you the website/Klaviyo admin define. You may need to collect information from the user that may not have a predefined slot in Klaviyo. This could be data such as “TeaDrinker” : Yes | No which you could use to track whether a subscriber drinks tea or not. This can help refine marketing experiences for you and your target audience.
All you need to do select the form field you want to map and then type a name to identify this data in Klaviyo. The form field can be any type.
In Klaviyo the data will be presented like this in the custom properties section.
Note: Do not add any space to your custom property keys (custom field name).