Adding custom field to every subscriber

I would like to add a custom field to every subscriber. This custom field is a 16-letter entrance code that logs a user into my page when he/she clicks it.

I like to set it through the API and then use this field in the email that is send to the subscriber.

I see a column 'custom_field' in subscribers, but can't find any documentation or way to initialise it and fill it with the API.

Is there any documentation? Or can you give me some indicators of how this works?