How to link Sales order with the Purchase order manually to create the reservation?

Summary:

We need to link sales orders with the purchase order to create the reservation against the demand document.

Content (please ensure you mask any confidential information):

We are trying to link the sales order with the purchase order but unable to associate the documents for creating the reservation. Once the PO is received, we could see the reservation can be created with the on-hand balances (as per the below screenshot). We need to create the reservation with the purchase order created for the sales order. Please suggest on the steps for linking the SO with the PO manually and proceed with creating the reservation. Thanks in advance.