If your computer has a Windows operating system (not available on Macs) then you can add “items” on the fly in the Desktop Edition.
For example, if you want to add a new inventory item while being in another tab, such as Purchase Order, then point at the Inventory Item tab, right click and select Open in new window, the window opens with the Inventory Item tab displayed. Create the Inventory Item, then click back on the Manager programme and the Inventory item will be available for selection.