You don’t need to know what the animation is yet. A rough cut, a treatment, or a paragraph about the thing you can’t film is enough to start from.
Earlier is better than later. Most of the animation I’ve made came out of conversations that started while a film was still being worked out. I’ve come in at the end plenty of times too — it’s just a different job.
If you’re a student, or it’s your first film, or you’ve never commissioned animation before, write anyway. I was taught by people who made time for me.