Here's some ReaScale guides:

Have a look at Kenny's video https://www.youtube.com/watch?v=dRN5NToxBlw

and you can use reascale also to snap to chord https://reaperblog.net/2012/03/how-to-use-reascales/

https://stash.reaper.fm/v/8075/ZD-Ultimate.reascale

also if you install https://reapack.com/ then go to Extension menu > ReaPack > Browse
Search
Right click > Install > Apply
mpl Snap takes to scale
you will then find it in Actions > Main

Right click > Install > Apply
mpl Snap selected notes to scale
you will then find it in Actions > MIDI Editor (of piano roll editor)
or Force selected notes into key signature