Hub bench planning is completed the same way as it is for standalone sites.
However, a Hub can only begin planning benches once all sites have set their productions or the cutoff time has passed for that production.
Once either of these events happens you will be able to click 'Plan Benches' and be taken to the bench planning page where you can plan your benches.
Once you have begun planning your benches for a production you will no longer be able to make edits to the production numbers.