I have this same Honda motor as the OP, bought new (well, the 100 hp version) and can confirm 100% that you attach to the hose for flushing and do not run the motor. I think other posts have already established that so just adding that is the info direct from the manual. I'll add my own question/problem to the debate about flushing that way or running on muffs though. I get inconsistent flow when flushing on muffs. Sometimes it seems to work fine, other times I get nothing from the pee hole. Have gotten the high heat alarm a couple times flushing with muffs and running.
So lately I have just been doing the passive flush through the hose fitting. I figure a passive flush with Salt Away is better than potentially overheating the motor on a regular basis. Still, for the reasons mentioned here I'd prefer to do a running flush if I could do it consistently. May try the tub of water thing but its certainly less convenient than muffs. Nothing has changed with my muffs or my water pressure, and I have no problems with the motor pissing actually on the water, have never overheated even running on step for 1.5 hours straight etc. Just seems to be on the muffs.