As explained by Asgardao and Woodcreat studio already - with direct monitoring on, all your audio that is being recorded is monitored via your hardware and can´t be heard in Cubase until playback. Therefore it can´t also be taken as send feed.
You can split input of your interface and send it out to a hardware reverb unit via your interface - if it allows such a routing via inbuilt mixer or something.
As said in your other thread already - with direct monitoring off and “constrain delay compensation” on and the verb as send, you can monitor through the low buffers of the soundcard, with the Plugin delay of the reverb not taken into account.
And lastly, though you don´t seem to answer my questions anyway and I know, it´s not what you´re asking - once again: The MR 816s inbuilt Reverb should provide you with direct monitoring and zero latency reverb.