One of Microsoft To-Do’s features is an integration with Outlook Tasks. To view your Microsoft To-Do tasks on your Outlook Desktop client or on Outlook.com, you simply need to use the same Microsoft Account to log in to both Microsoft To-Do and Outlook. All tasks are stored on Exchange Online servers, so they will be shown in both Microsoft To-Do and Outlook Tasks automatically.
Please know, however, that Microsoft To-Do doesn’t yet support all the varied features of Outlook Tasks. While data you have entered in those additional details in Outlook Tasks will be stored safely on the servers, it will not be displayed in Microsoft To-Do. Specifically, Microsoft To-Do does not currently support the start and end dates, task status, task completion percentage, multiple priority levels, task work hours, task colors, option to format text in notes, or file attachments that are available in Outlook Tasks.
We are currently working to deepen our Outlook integration. Please tell us which options you miss most so that we can prioritize those features with our next updates.