When raising an order you can choose a product from the drop-down list, provided you have first selected a Supplier. Just choose the product and click the + Add Product Line sign next to it as shown
If you add a product, a blank line item will remain. This can be deleted if not required. Click the blue cross at the end of the line. If you compose a line item and add a new product code, this will be saved as a product against the supplier – provided your admin has enabled this feature.