The vocal swell before the singing starts sounds to me like a reverse reverb swell ... here's a basic tutorial (for pro-tools, but you can see how it works):
https://www.youtube.com/watch?v=s8zAh8hK0Gc
This is how I would do it:
- Cut off the first word of the vocal onto a new track.
- Reverse (right-click item, "reverse item as new take")
- (I usually will then right-click again and glue track, because seeing multiple takes annoys me, but this is not necessary)
- Add a big fat reverb with a long tail
- Render to new track (right click on track, "render item as stereo stem track and mute original" - you can delete unused track now)
- Reverse item again
- Chop item so you only hear the reversed reverb tail
- Fade in reverb, and place right before vocal track