How do you get anti form on Kingdom Hearts

From my experience, you can only get anti-form by a random chance. It's kinda like how you get Final form, only you can choose Final Form after you get it the first time. My advice is to randomly go through the forms and after awhile, you should come across it. But no, there is no way to choose anti-form at any time. It's a random thing. Heartbreaking, I know.

:(