Unfortunately, Apple currently supports a very limited list of "functions" and apps via Siri, and to-do apps aren’t on it. We expect Apple to add support for more domains/apps soon, so that we can integrate with it.
As a workaround, you might want to try using the following IFTTT recipe:
Simply ask Siri to set an iOS Reminder to send a task to your Todoist Inbox with a label @ToTask and a deadline for the following day. You can later view your task in Todoist Inbox and move it to a specific project or reschedule it accordingly.