How are things, MJK Design Group?

I can see how ideal to link the Purchase Order to the estimate within QuickBooks Online. However, what you've figured out about linking between these transactions is right that we're unable to link the PO to the estimate.

Yes, you're right that we can create a PO within the project but, it won't link to the estimate. One of the reasons is because we normally use different transactions for customers and the vendor's side. Let me also share what I know how to estimate in QuickBooks Online works. Once an estimate is saved in QuickBooks, that transaction can be turned into an invoice if you decide to accept the estimated price. This can be used on the customer's side. 

Instead of linking the PO to estimate, we can create billable expenses to invoice. For more tips and details, you may want to check this article: Enter billable expenses.

I'm always here if you require additional questions. I hope you're well and safe!