Is it possible to send notification to a purchaser to create Purchase Order?
as suggests by the experts, planning worksheet is the solution. If you don't have premium license, I guess it should be requisition worksheet for the same feature.
the tricky thing is this requires some planning and complexity to set up and test.
a simpler feature is Planning function inside a sales order, but those are not recommended to use with planning mentioned above. Simultaneous use of different planning tools may confuse BC.