Default Tor Browser Bundle's behavior is
you -> proxy -> tor -> website.
And this is not what you want.
That's why you shold not use your Proxy Settings from Tor Browser Bundle. You should torify the whole browser instead, despite
"torify" applications like Mozilla Firefox is no longer recommend
(from Torify Howto
). Not recommended but still works.
The other way is to use Virtual Machine that transparently routes all it's traffic to Tor. See whonix.org for details - this is probably the most secure solution available.
In the Virtual Machine you will have everything already "torified", the whole guest OS. So if you put in browser your private proxy settings, you'll get exactly what you need: you -> tor -> proxy -> website.
The third way is to set up manually TUN-interface that routes everything to Tor, and use it to access internet. But this is exactly the same that 'torify' does.