Hi,
You can still enter the field name, even if it does not show up – the list is only to suggest field names. Only fields that have been assigned values can be displayed in the suggestions list, the plugin looks for fields within the metadata table. The most likely reason you can’t see it in the list is, that it has not been used yet. Once you assign a value to it within a post, the field should start to show up, that is likely the reason for the other fields showing up eventually too.