Assuming you are using platform movement, untick the option to use default controls. Create a boolean flag in the player called "direction". Set up your player movement something like:
If key pressed (whatever key you choose for moving left) and boolean "direction" not set, then simulate platform pressing left.
Do the same thing for moving right.
Then set up a collision event between the player and the object that sets the boolean flag for "direction". You would then need to put in an event like:
If key pressed (whatever key you choose for moving left) and boolean "direction" set, then simulate platform pressing right (this reverses normal movement). Do the same thing for moving right (which reverses movement so the player moves left).