Hi Roy,
From what I can see in the manual, the USB port for the Samsung HW-C400 is for media playback from a USB device like a flash drive (source):
Additionally, pages 22 & 23 confirm the USB port is for a storage device not any sort of control and so I don’t believe that is correct.
The USB on the TV marked for service is just that. It’s for service, so doing things like potentially updating the firmware of the TV, or for technicians to diagnose issues.
From what I can see, page 16 would indicate the only change that needs to be made, is setting the TV to use an external speaker, and then it should work provided the TV supports this. This is going to be because the TV itself, is now outputting all of its audio only to the soundbar/receiver.
Now, with all that said, I have spotted the issue. Using an external remote with the soundbar requires setting the TV to an external speaker, however, your TV, disables the volume buttons when set to External Speaker, as per page 27 of the manual (source).
As such, it’s not possible to use the TV remote to control the soundbar. You would require a different soundbar or a different TV.