Code:
// on error = my.x - 50 != behind pos
// example only: This code SHOULD NOT work.
var behind_v[3] = {0,0,0};   // reusable

action cube_fall() {
   //Put random physics code or whatever you want the cube to do in here, this will be the cube's behavior.
}

action player_a() {// Behavior assigned to the player
	var kin_create; kin_create = 0;
	while(1) {
		if (key_enter == 1 && kin_create == 0) { //& not pressed last frame
			vec_for_angle(behind_v, vector(ang(my.pan - 180), 0, 0));
			vec_scale(behind_v, 50);
			ent_create("cube.mdl", behind_v, cube_fall); // creates the cube 50 quants behind the player and assigns its behavior.
		}
	}
	kin_create = key_enter;	// key_enter pressed last frame?
	wait(1);
}