Thanks for that link — Games.
I tried the game out, but it looks like the two slopes in there are both under the 45degrees mark.
Actually, I am getting close to a solution. I use the on collision with the sloped platform as a trigger. Than check whether to see if the angle is between the angles I am looking for. And also to check if the correct key is pressed, I perform the sprite move at angle action. I set the angle to the same one as the platform, and + or - depending on the movement direction.
It works well to certain extent. It moves only once, so I need to continue to press. I can't use the on key press down because it is a trigger and I already have one trigger with the collision detection.
I am going to try the bullet option next. Hopefully I will be able to find a workaround to this.