When performing a Production Run from a Production Item Template that includes Optional Items, the Optional Items button is available and highlighted by default.
Select the Optional Items button to open the Optional Items screen.
Select and press [Enter] on an Optional Item to add it to the current Production Run.
Optional Items added to the current Production Run are temporarily removed from the current Optional Items screen and added to the current Inventory Production screen (but will be available for selection from the Optional Items screen during future Production Runs).
See:
Production Module Contents