To use your Apple Watch with Microsoft To-Do, you can connect your Outlook.com or Exchange account. Just head to your iPhone's settings, select Accounts & Passwords and then Add Account. Then select Exchange or Outlook.com and simply follow the prompts to add your account.
Once you’ve added your account, you’ll also need to enable Reminders for that account. Just select Accounts & Passwords once more and then select either Exchange or Outlook.com. Then make sure that Reminders is toggled on for that account.
Now your tasks from Microsoft To-Do will sync to Reminders, where they’ll also be visible on your Apple Watch.