The pre-requisite for using Siri with To-Do is that you must have registered your Microsoft Account with iPhone. If you have a work account then you need to register your Exchange account with iPhone. Now fire up iPhone's settings, tap Reminders > Default List and then select the desired list. Now you use the voice commands to add reminders or notes to To-Do. Just say ‘Hey Siri, remind me to…’ and your reminder will be synced with To-Do.