Due to a bug in Tor Browser,  extra steps are required to use proxies.
Note: This action will break both the Stream Isolation for Tor Browser and Tor Browser's tab isolation by socks user name. This worsens the web fingerprint and causes the user to be pseudonymous, rather than anonymous. To mitigate these risks, consider using More than one Tor Browser in Whonix, or better yet, Multiple Whonix-Workstations.
Complete the following steps inside Whonix-Workstation.
1. Install the FoxyProxy add-on in Tor Browser.
2. Change Tor Browser Settings.
Use Proxy "Default" for all URLs