You can right click the empty slot to search for an item by name to easily set the stock item or press the button to choose an item. Multiblock components are special so they won't be found by a normal search instead search for multi, placement, or component for a list of these blocks.
The port's internal inventory can be locked using the labeled button in the interface to prevent the port from collecting items if the stock limit has been reached. This function is intended to enable limiting the backlog of high value items.