In this example he uses a speed variable that is worth 400. It would be the same as if he had said set y to TiledBackground.Y + 400 * dt. (Without the variable). So I hope you noticed that there is a global variable at the beginning of the event, if you didn't create the variable, nothing happens. You can just do it without the variable as I informed above. And you can use self in place of the object name, which is an expression that refers to the object itself, without having to type the name.
It would look like this: set y to self.Y + 400 * dt (I tested it and it worked). And note that the tiled background must be twice the size of the layout (Always for this method), because when it reaches half the screen it goes back to the beginning. / But I said all this just for you to understand the operation and not just decorate.
Simply use a tiled background the right size of the layout and in Every tick - / Tiledbackground - set image y offset- Self.ImageOffsetY + 400 * dt condition
What is it? At each tick, select the tiled background object and choose the set image y offset action, which basically moves the tiled background object in the direction you want to repeat endlessly. It scrolls the image within the image itself, without needing to have an image twice the size. Then the code means self (the object itself and imageoffsety + add 400 to that scroll.