Purchase orders (POs) can be created manually however, they’re more typically generated from other processes. Orders can be automatically created based on other documents, such as a sales order.
In Dynamics 365
Dynamics 365 Finance and Supply Chain Management, Procurement and Sourcing fully supports the creation of Purchase Orders from an existing Sales Order on a back-to-back basis as standard.
Select or create a new sales order in Sales and marketing. On the Action Pane, click Sales order. Click Purchase order.
The Create purchase order page lists all the open sales order lines which have been copied from the sales order. You can review the order details, and if required, you can modify selected details such purchase quantity and pricing terms, before you create the purchases.
To review purchase orders created from sales orders, do this from the General tab in the Action Pane. The Related orders page lists the sales orders the purchase orders were created from.