Yep, as above, envelope controller attached to the volume setting.
My Envelope settings for this effect are ...
Threshold: -70dB
Attack: 350mS (make this longer for a slower swell)
Release: 1mS
Gain 2.5
Just like the real pedal, you need to mute everything to get a swell on the next note(s). I thought an ADSR might work better for detecting the pick of new notes, but couldn't really find any settings that worked better than the envelope controller.