Custom fields can be used to capture additional information link to the customer or a specific subscription.
To know how to create custom fields Click Here.
These custom fields can be configured to be Hidden, View only, Mandatory & Editable.
To pre-fill the custom field in the hosted page you will need to use the custom field ID, which can be access from API documentation.
Sample URL format;
If you are using our APIs + Checkout pages, you can pass the custom field value through the API call.