I also did this by accident.
Horse used to live in a field at the top of a hill. The field gate opened onto the road, and across the road was the stone I used as a mounting block. I used to tack up just inside field gate, then open gate and lead horse out. Over time, when I led him out, he got used to standing waiting for me to fiddle about with the gate before heading over to the mounting block. Eventually (don't report me folks), I used to just open gate and invite horse out, and throw the lead rope over his back while doing the gate. It then occurred to me what he was doing, so I added a cue (say "stand", throw rope over back...) - and then started to try doing it at other times.
It's not the usual ground-tying - throwing the lead rope over his withers is the cue, rather than letting it dangle. I suppose I could go back and train "drop rope then say stand..." but what I've got works, so why bother?
The key to it is, if you've got the behaviour somewhere and it's reliable, just add a cue immediately before it happens...
I've trained "stand loose for grooming and tacking up" as well, but used clicker training for that one.