Put this in a code block in the first scene of your game in a when created event:
untyped __js__('var keys = {};
window.addEventListener("keydown",
function(e){
keys[e.keyCode] = true;
switch(e.keyCode){
case 37: case 39: case 38: case 40: // Arrow keys
case 32: e.preventDefault(); break; // Space
default: break; // do not block other keys
}
},
false);
window.addEventListener("keyup",
function(e){
keys[e.keyCode] = false;
},
false);');