Command Line for Enable Dragging
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): 

com.apple.driver.AppleBluetoothMultitouch.trackpad.plist
com.apple.AppleMultitouchTrackpad.plist


They contain the following relevant keys:

&lt;key&gt;DragLock&lt;/key&gt;
&lt;false/&gt;
&lt;key&gt;Dragging&lt;/key&gt;
&lt;false/&gt;
&lt;key&gt;TrackpadThreeFingerDrag&lt;/key&gt;
&lt;false/&gt;


so you can set these keys directly with defaults:

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.