You would have to run it through replenishment to create a PO without using the create PO function on the Sales Order. 

The item doesn’t need parameters (Safety stock, min/max, reorder point) but it does need a replenishment class of purchased associated with it. Check the item class replenishment setting is set to Item class settings and as long as your availability calculation is set to deduct qty on sales orders replenishment will see it as demand. 

Hard demand would only see SO demand for items that were allocated to the SO.

Then when you go to the prepare replenishment screen it will only recommend you to purchase the qty of items needed for that Sales order. 

If we add an Item Class Replenishment setting, would if affect the items in this Item Class that do have replenishment settings that we need to keep?

@scottaisd00 I’m pretty sure the Demand calculation setting will effect existing items. For this scenario you would want to add the purchase replenishment class to just the stock item profiles that need it. 

Try setting an item that is on a SO that you want to purchase for using these settings and go to the prepare replenishment screen and refresh it. 

You should see the amount you are short according you your availability calculation show in the qty to process field on the prepare replenishment screen.