Task management app by Microsoft. Based on Wunderlist. https://to-do.microsoft.com/
As far as I can see, the To-Do app and website are a front-end for the Outlook.com/Exchange tasks. I’ve got Outlook (desktop app) connected to my account and everything appears in there as a task, so this is probably possible via the Exchange API.
It’s included as part of the Microsoft Graph API now: https://docs.microsoft.com/en-us/graph/api/resources/outlooktask?view=graph-rest-beta