It is also possible using Dual Mode:

Guitar > H90 input 1

H90 output 1 > Opus input

Opus output > H90 input 3

H90 output 3/4 > Stereo amp/monitors

I would suggest using Insert mode instead since it is more flexible; you can put the Opus before Preset A or after Preset B. This would not be possibly in the Dual Routing configuration. But I encourage you to try it either way and see what you like.

Make sure you are switching the Global Routing mode from Insert to Dual when you are experimenting with this.