It's not so much a curve, it's just perspective. The simplest kind is x/z and y/z, where z if how far into the screen the ball is.
Give the Ball sprite some instance variables called "z", "startX" and "startY". Also you presumably you have the two sprites horizontalSlider and verticalSlider. The events below assume the ball is launched as soon as the layout is run and the sliders are already in place. All the numbers are tweakable.
global number t=0
ball: on created
--- set startX to self.x
--- set startY to self.y
--- set t to min(t+dt, 1)
--- ball: set z to lerp(1, 10, t)
--- ball: set x to (lerp(self.startX, horizontalSlider.x, t)-screenwidth/2)/self.z + screenwidth/2
--- ball: set y to (lerp(self.startY, verticalSlider.y, t)-screenheight/2)/self.z + screenheight/2
--- ball: set size to (64/self.z, 64/self.z)