The Headphone output of the AudioBox 1818 VSL mirrors Output 7 & 8.
So any signal routed to Outputs 7 & 8 can be heard from the Headphone Output.
You may find the following links helpful to get audio routed to Outputs 7 & 8 in Reason:
https://ask.audio/articles/setting-up-a-monitor-mix-with-a-click-track-in-reason