If a Purchase Requisition is created from a Buy List, products with quantities should show at the top of the Purchase Requisition by default.
However, if the Buy List has been set up with the [Custom Sorted] box ticked.
For more information, check out this article, Create a Buy List. The product positions in the Purchase Requisition will be shown in the same order that they are in the Buy List.
The products with quantities will show on the top positions once the Purchase Requisition is submitted for approval.