What is the base URL for QuickBooks Desktop?
Per the documentation, the URLs for IDS for QuickBooks desktop follow the format:
https://services.intuit.com/sb/<objectName>/v2/<realmID>
However, the problem you're having:
I see my QuickBooks Online instance only.
Doesn't really sound base URL related. It sounds more like you haven't set up Sync Manager, or that your app settings don't have the QuickBooks desktop options ticked off to allow desktop versions of QuickBooks to connect to your app.