This reply from the admin does not explain how to upload a vendor/contractor's W-9 form to QuickBooks. It just tells you how to enter the Vendors name, address and EIN (if they have one). 

This is from the "Admin" at Quickbooks:

To add the vendor's W-9 in QuickBooks Online, you can enter it manually. Here's how:

1. Go to the Vendors menu and then select Vendors.
2. Click the vendor's name, then Edit.
3. Enter the Company, Complete name, Business ID No.(Tax ID No.), and address.
   
4. Click Save.