Events live in the us.timinc.mc.cobblemon.catchondefeat.event package, and can be accessed via CatchOnDefeat.Events.
JOIN_ON_DEFEAT_PREFired early on when considering a Pokémon attempting to join the team. This event can be canceled. Doing so will result in the join on defeat not going through, and the player will not be shown the confirmation prompt.
pokemonThe Pokémon attempting to join the given player’s party.
playerThe player whose party the given Pokémon is attempting to join.
JOIN_ON_DEFEAT_POSTThis event is fired off when a join on defeat attempt succeeds. This means the player has already accepted the join as well.
pokemonThe Pokémon that has joined the given player’s party.
playerThe player whose party the given Pokémon has joined.