question

Any inventory of a warehouse is a result of entering items in it. You can transport items slowly to a rack by Taskexecuters or you put them in by a move object command or a process flow activity.
I suggest you try to let enter a single item in your warehouse in a slot of a rack object. You find a warehouse demo model, example models to fill racks by label, level, bay or other conditions already demonstrated here at this answers site.
If you get one Item into a rack, then you do this method several times for the amount of items you want to have as an initial stock. The initial stock is a result of actions done at the runtime of 0.0000. That is central point.
Sometimes you must let finish a code sequence before you proceed to another sequence then you insert a delay process flow activity by 0 time units. This delay of 0 resolves all pending source code actions. You find it often as a breathe activity in example models.
Working with initial stock later depends on the fact if you want to store any data in an own global list or to store any relevant data in labels on your items in the warehouse.
if you prefer a global list then you push the item reference of the token which created the item onto the list with all your intended data.
If you have stored all relevant data in labels at your items, then you can query this data later to identify your items.
Warehouse Demo Model

Fill rack by condition painted slot / level