Assigning a Warehouse Interface with "Z" as a workstation in the Work Order seems to be working (at least the picture and coordinates appear in the list).
Giving the Work Order too a Craftsman makes him go too the coordinates and plays the working animation but the worksite itself is not getting worked at (items stay in the Warehouse Interface).

How to recreate:

1. Let a Courier fill the Warehouse Interface
2. Select the Warehouse Interface as a worksite with "Z" in the Work Order
3. Give Work Order too a Craftsman

Taking the Work Order away from the Craftsman solves the problem but he has to choose the Warehouse Interface himself for it being worked on.

First off, please join our discord for quicker problem solving: https://discord.gg/ebZskE7

To answer your question, the Warehouse Interface is not what need to be worked on, the Warehouse Control Block is. A Craftsman working at the Control Block will move all items into and out of the Interfaces. However, I recommend using a Windmill to power your warehouse since it can be quite power intensive.