Hello. I would like to propose a solution that worked for me.
In Main scene script I would write:
func _ready():
screen_size = get_viewport_rect().size
# Clear all Vector2 points
$MobPath.curve.clear_points()
# Set 1st vector
$MobPath.curve.add_point(Vector2(0,0))
# Set 2nd vector
$MobPath.curve.add_point(Vector2(screen_size.x, 0))
# Set 3rd vector
$MobPath.curve.add_point(Vector2(screen_size.x, screen_size.y))
# Set 4th vector
$MobPath.curve.add_point(Vector2(0, screen_size.y))
# Set 5th vector
$MobPath.curve.add_point(Vector2(0, 0))