Hey folks,
Probably not terribly needed as a guide but this is doable as soon as you exit the Javelin for the first time with a coop partner. Simply walk up to them and mash X a couple times to slap them and it will pop.
Note that as funny as it is to beat up your coop partner it's actually a good way of getting health back in a pinch. If your partner has ~one bar of health left, just slap or shoot them into a downed state and when you slap them back up they'll have full heath, similar to Rainbow Six Siege when you knock a teammate and revive them to get them back in fighting shape.