To use Siri with Microsoft To Do, you can connect your Outlook.com or Microsoft Exchange account. Just head to your iPhone's settings and select Passwords & Accounts > Add Account. Then, select Exchange or Outlook.com and follow the prompts to add your account.
Once you’ve added your account, you’ll also need to turn on reminders for that account. To do that, select Passwords & Accounts, select either Microsoft Exchange or Outlook.com, and make sure that reminders are toggled on for that account.
Once your Microsoft account has been connected, you'll need to set your default reminders list to one in your Microsoft Exchange or Outlook.com account. Open your iPhone's settings, tap Reminders > Default List, and select the desired list.
Once you say "Hey Siri, remind me to...", it will be synced with Microsoft To Do. Note that Siri does not currently work with business customers who use InTune.