scmJS docs

Set Score

Sets the player's score of the kind, adds to it or takes from it. Custom is the score maps keep for themselves, often shown with Leader Board Points.

Action 27 · Set Score(Player, Modifier, Amount, Score);

Argument Takes Stored in the action
Player a player or group bytes 16–19
Modifier Set To, Add or Subtract byte 27
Amount a whole number, 0 to 4,294,967,295 bytes 20–23
Score a score type bytes 24–25
Set Score("Current Player", Add, 10, Custom);