Todoist does not offer free trials of their Pro plan. However, if you are unsatisfied with your purchase after trying out the Pro Plan ($3/month) you can request a refund within 30 days.\
Todoist offers free 30-day trials of their Business plan. You can begin your free trial by tapping Try Free on this page.
Mentioned in this answer
Todoist
Organize your life. Then go enjoy it.