The easy way to find the plist key for a given preference is to open a Finder window of ~/Library/Preferences, set it to List View, sorted by Date Modified.
Then, set the preference and see what file gets modified.
From that, I get two almost identical files modified (on 10.14.5):
They contain the following relevant keys:
so you can set these keys directly with
defaults write com.apple.AppleMultitouchTrackpad Dragging -bool true
defaults write com.apple.driver.AppleBluetoothMultitouch.trackpad Dragging -bool true
You can include three-finger dragging or Drag Lock, if that's what you want with similar commands.
Finally, run the following for the writes to take effect: