From what I understand, the "csv" prefix is for standard built-in WP fields. When this plugin encounters a column header that doesn't begin with "csv" it assumes it's a custom field and adds it as such.
Therefore, the column heading needs to be the meta key for the custom field.
There's no standard prefix for the meta key as different plugins use different prefixes to avoid conflicts. For example, Yoast SEO uses "_yoast_wpseo_" as their prefix. Custom fields created by the Types plugin have "wpcf-" as their prefix.
There are a couple of ways to determine the correct meta key:
1. Go to an entry (post, page, CPT) that has a value for the custom field that you want. Make sure the Screen Options has "Custom Fields" checked. See if there is a listing of Custom Fields Name (this is the meta key) and Value.
2. Look for the value in the postmeta table of your database. This usually involves going into your webhosting account and using something like PHPMyAdmin to view the database directly.