Sidechaining is a bit unfinished. Short answer is it depends on the plugin, some plugins report a special "Sidechain-channel" to the host. this is not implemented yet. Other plugins simply have an extra "normal" channel. with these you can do side chain now.
If you have a mono plugin with side chain, add a stereo chain, add the plugin, route the signal to input one and the sidechain to input 2.
If you let me know the specific plugin you want to use sidechain on I can have a look at making it work.
I see now that the VST3 version of C6 has double inputs. (Stereo has 4 inputs and mono 2)
So for stereo, add a 4 channel chain, route the main signals to 1/2 of the chain and the sidechain signal to 3/4