This may not be the best solution, and I don' t know much about your implementation to test it. But you should be able to determine velocity by comparing previous position with the current position of an object. It is usually something used in Verlet integration.
So in your platform, assuming it is a Kinematic, you can have some code like this:
var prev_pos = get_pos()
var velocity = Vector2()
velocity = get_pos() - prev_pos
prev_pos = get_pos()
At which point when you have a reference to it you just check it's velocity property.
player_velocity += platform.velocity
Big note: You may need to do some additional math with delta, depending on your setup.