Here's an example (Godot 4.x):

var collision_shape: CollisionShape2D = CollisionShape2D.new()
collision_shape.shape = ConvexPolygonShape2D.new()
collision_shape.shape.points = PackedVector2Array([\
    Vector2(0.0, 0.0), Vector2(1.0, 0.0), Vector2(1.0, 1.0), Vector2(0.0, 1.0)\
])
add_child(collision_shape)