It's mentioned in my FLHRSE4 owners manual but doesn't tell how to turn it on or off. Actually it says see the Dealer . . .
A buddy told me you roll the throttle forward (off) and hold it until the cruise control light comes turns RED, release the throttle and then roll throttle forward again and hold until cruise control light turns green.
I have not tried it so take it for what it's worth.

It has been quite some time since I read in my electrical manual for my '08 SERK. I had all my electrical manual WAY before I had my bike and I got the shop and supplement afterwards. So from memory, and this will be close and in theory correct. Once a certain temperature is reached, temp not given or told where it is monitored, the rear cylinder cuts off and the idle drops from 1,100 to 900 rpms. As long as you stay below 7 mph it will stay in the "Parade Mode". Once you reach 7 mph it ignites the rear cylinder and away you go.
There are lots of times here in St. Louis lately that when I'm stuck on the freeway in the great bike parking lot, I turn off the bike but then have to restart it. If I could roll forward the throttle assembly to "put" it into Parade Mode, I think that would be a big help. At least it would make me feel better. Maybe there will be a new firm ware flash that they can retro back to the '08s. I can't believe we don't have the hardware to do this already on the '08s.
I know some of the guys have replaced their horns with the cooling fans and they say that does help keep the temps down while stuck in traffic. Yes, they reposition the horns, I didn't mean they removed them entirely....
Hope this helps.