Prototype definition:

data:extend({
{
  type = "custom-input",
  name = "my-custom-input",
  key_sequence = "SHIFT + R",
  consuming = "script-only"

  -- 'consuming'
  -- available options:
  -- none: default if not defined
  -- all: if this is the first input to get this key sequence then no other inputs listening for this sequence are fired
  -- script-only: if this is the first *custom* input to get this key sequence then no other *custom* inputs listening for this sequence are fired. Normal game inputs will still be fired even if they match this sequence.
  -- game-only: The opposite of script-only: blocks game inputs using the same key sequence but lets other custom inputs using the same key sequence fire.
}})

Local definition:

[controls]
my-custom-input=Potato controls

Subscribing to the event:

script.on_event("my-custom-input", function(event) game.players[event.player_index].print(event.input_name .. " ran on tick: " .. tostring(event.tick)) end)