Currently, Apple only supports a limited list of “functions” and apps via Siri. Unfortunately, this doesn’t include to-do lists (and therefore Todoist) at this time.
Nevertheless, there’s a handy workaround via IFTTT (If This Then That) that can help until Apple chooses to support to-do lists.
First, create an IFTTT account via www.ifttt.com.
Next, click “add” on the IFTTT “applet” below to turn on the connection between Todoist and iOS Reminders.
With this applet activated, every time you create an iOS Reminder via Siri, a Todoist task will also be created.
Inside the “applet” in IFTTT, you’ll be able to select which Todoist Project your tasks will be added to and which priority they will have.