One of Microsoft To Do's features is an integration with Outlook Tasks. To view your To Do tasks on your Outlook Desktop client or on Outlook.com, use the same Microsoft account to sign in to both Microsoft To Do and Outlook. All tasks are stored on Exchange Online servers, so they'll be shown in both Microsoft To Do and Outlook Tasks automatically.
Microsoft To Do doesn't yet support all the varied features of Outlook Tasks. While data you enter in additional details in Outlook Tasks will be stored safely on the servers, it won't be displayed in To Do. Specifically, Microsoft To Do doesn't currently support the start and end dates, task status, task completion percentage, multiple priority levels, task work hours, task colors, or categories that are available in Outlook Tasks.