Hi Max,
Try this one here:
https://outlook.live.com/calendar/0/deeplink/compose?body=Description&enddt=2021-02-17T16:45:00+00:00&location=Location&path=/calendar/action/compose&rru=addevent&startdt=2021-02-17T16:15:00+00:00&subject="Title Of the Event"
Change the values as your needs and put it as the External URL of a link. I suggest that you encode the url (there is an OutSystems function for this)
I have made a test here and it works. You can also omitt values if you want.
Hope it helps