Welcome to the Community. I can share some information about the feature you need in QuickBooks Online.
Currently, an option to enter blanket purchase orders (PO) is unavailable. As a workaround, you can use the Estimate feature to create Progress Invoicing (allow you to create multiple partial invoices from a single estimate) and also be able to convert an estimate to a vendor PO if needed.
Right now, these are the only options you can use to handle blanket POs in QBO. I can see how this ability will help users with tracking multiple delivery dates over some time.
We continue to improve QuickBooks Online, and this preference might be added in the future. For now, I suggest you visit our QuickBooks Blog to stay updated with all the changes that are being made.
Also, I've attached some articles you can use to learn more about entering estimates, as well as using progress invoicing in QuickBooks: