When you push a purchase order from Inventory Planner to Quickbooks Online, Inventory Planner creates an identical PO with the same products, quantities, cost that you see on the PO dashboard side.
Before saving a PO you need to make sure that all products are created in QB. Otherwise you will get an error - Inventory Planner doesn't create products in QB automatically.
Note that this integration works with Quickbooks Online, but not Quickbooks Desktop.
Go to Account > Settings > Purchase Orders and you can enable saving POs to Quickbooks.
Then after you've created a new purchase order, you'll see the option to Save to Quickbooks.