QuickBooks Online (QBO) is dependent on the data shared by your third-party application, such as Approval Max. Though I know a workaround you can utilize so you can add a buyer's name field in the Purchase Order. Let me walk you through the steps how:
In your QBO account, go to the Gear icon, then select Custom fields.
Click on Add field. Note: If this is your first time creating a custom field, select Add custom field.
Enter a name in the Name field (e.g. Buyer Name).
Tick the Purchase Order checkbox. Tip: To show the custom field on printed and delivered forms, turn on Print on form. Otherwise, it’ll only appear in QuickBooks Online.