Make the player an immovable physics object as well. And just move it with the platform movement. Or make your own platform movement for the player out of physics operations. If you use the first method, you'll notice the boxes move out of the way, but with no real force. To fix this, you can give your immovable physics player commands like
always -
----set physics horizontal velocity to. Platform horizontal velocity * factor
I don't think those are the actual expressions, but I don't have my pc in front of me. Factor will be a number of your choosing, expirement till it seems right. Even though you won't actually be moving the player with physics, this will let the physics engine know the object is moving
I have a potential fix for physics that will make it work naturally with outside movement without the extra steps. Hopefully it will make it into a future version of construct