The syntax for the scroll_anim function is
scroll_anim <xspeed> <yspeed>
But FH doesn't use the X axis, so that value does nothing, and Y is the value to make it move up and down.
However, the FH UV is not a strictly up and down texture, so it usually comes out sideways unless you can somehow get it to match up...
It's tricky.
The closest I've come is by using this texture
and setting the scroll values to 1.
But maybe someone else knows better than I.