Hi,
Replenish on order, also known as MTO (make to order), is a replenishment strategy that creates a a request for quotation (RFQ) for a product every time a sales order is created for it. The creation of a RFQ  occurs every time a sales order is created, regardless of the current stock level of the product being ordered.
1. Go  to Inventory ‣ Configuration ‣ Routes. On the Routes page, click the Filters button and select the Archived option. This shows all routes that are currently archived. and unarchive the MTO route.
2. Inventory ‣ Products ‣ Products, then select an existing product, or click Create to configure a new one.
On the product page, select the Inventory tab and enable the Replenish on Order (MTO) route in the Routes section, along with one other route.
3. If the product is purchased from a vendor to fulfill sales orders, enable the Can be Purchased checkbox under the product name. Doing so makes the Purchase tab appear alongside the other settings tabs below.
Select the Purchase tab and specify a Vendor and the Price they sell the product for.
4. Sale-> create sale order-> confirm,A Purchase smart button now appears in the top-right corner of the sales order. Clicking it opens the RFQ associated with the sales order.

Hope it helps,