How can I create Todoist tasks with Siri?
For now this feature is only available for Reminders.

In iOS 11 it's possible for todo list apps to use Siri. So you need to wait for iOS 11 and for the developer of Todoist to update it's app to add support for Siri.


  SiriKit gives developers the ability to integrate Siri into their
  apps, and with iOS 11 it expands to support more categories, including
  to-do lists, notes and reminders; banking for account transfers and
  balances; and apps that display QR codes.


Source: look for SiriKit