You are not adding the floor_velocity to the movement vector, also, just do that only if colliding (and get collider velocity only if colliding).
Moving platforms, in general, are between the 4th and 7th circles of hell so, be patient because you may face a some issues depending on your game mechanics.
I suggest you to do some little tests first, create a simple body affected by gravity vector only, then create the platform, move the platform and try to make the body follow the platform displacement, then apply user input and see how works until you find the motion effect you want.
Sorry for bothering you with the same subject.
I can not catch the speed of my platform and move to my player.
I know there is a function called get_collision_velocity, But I have no idea how to use.
There is no tutorial on the internet speaking or teaching about it!
The only thing that has it’s a demo and it’s complex.
It’s frustrating to see things not working.