To do this, you will need to import an Illustrator file into ProAnimator. For the Illustrator file, prepare each layer with the shape of the object. You will need to change the shape of the object in each layer. After setting up the Illustrator file, save the file with no compression.
Then, in ProAnimator, import the Illustrator file and be sure to enable the option "Open By Layers". What this will do is it will create an object and each frame in ProAnimator will correspond to each layer in Illustrator. As you are playing the animation frame by frame, the ProAnimator object will change accordingly and create the "write on" effect that you're looking for. So you can imagine this similar to artists drawing a different picture on each page and flipping through the pages quickly to create the effect of the animation.