To create a custom field, go to Subscribers and click Fields. Here you can see all of the available default fields.
To create your own, click Add new field. Give your field a name and choose the type of field.
Then click Create to save your new custom field. You can use these field in autoresponders and when collecting new subscribers.
Quick note: There is a difference between default fields and the ones you create yourself. You can rename or delete the fields you create. However, you can’t delete the default fields.