Sorry for the delayed reply ol' chap.
In this trigger, assuming player 1 is host player,
Events
Player - Player 1 (Red) types a chat message containing -ffa as a substring
Conditions
Substring((Entered chat string), 1, 5)) Equal to -ffa <-- so that typing "1234-ffa1234" will not work
Actions
Set killNeededToWin = Integer((Substring((Entered chat string), 6, (Length of (Entered chat string)))
so when player typed -ffa 34 then killNeededToWin = 34.
In Team Death Match Mode, to make the players treat each other allies, you can use this
For each (Integer A) from 1 to 6
Loop - Actions
For each (Integer B) from 1 to 6
Loop - Actions
Player - Make (Player(Integer A)) treat (Player(Integer B)) as an Ally with shared vision
For each (Integer A) from 7 to 12
Loop - Actions
For each (Integer B) from 7 to 12
Loop - Actions
Player - Make (Player(Integer A)) treat (Player(Integer B)) as an Ally with shared vision
this makes players 1 to 6 treat each other allies and players 7 to 12 also treat each other allies.