If you have created a separate Purchase Order for stock and want to allocate this stock before it has come in, you can use the Allocate from Future Stock button in Requirements.
You can only Allocate from Future Stock if the status of the purchase order is purchased and the line on the order is for stock. Ensure when purchasing, pre-book and auto receipt is not selected. It will not allocate if the status of the purchase order is stocked on Pre-Book as this means a receipt note has been created already.