If the codec is listed in the dropdown box it is also available for encoding. Voukoder tests all possible encoders on startup.
This line:
[17:37:40] Display #0 on: Intel(R) HD Graphics 4600
means only your monitor is connected to on-board GPU and not to the AMD GPU.
So, how do I use it exactly? Clicking OK just defaults it back to standard H.264 using CPU so I can't seem to use it. Drivers are up to date.