Hi Giuliano - Thanks for submitting a feature request! I think your request is similar to this one here.

In order for the fields to be populated in the schema tab, an initial sync needs to be started so we can identify what fields are present in your source. You can read a bit about this process in our documentation here. 

Fivetran starts an extract to fetch the schema from your source, which enables you to review it and block or hash selected columns before sending data to your destination.

Best,

Erin