You need to launch a protocol as a hyperlink in the web page, and then Outlook Calendar (if available on the user's machine), would launch by default to take control of the data processing request. The default URI scheme for Outlook Calendar is,
, so you need to write the following code in the button.
Your HTML content, precisely would be,
<button id="outlookCalendarBtn" onclick="window.open('outlookcal:')">
Open Outlook Calendar
This code requires Outlook Calendar, installation. If Outlook is not installed — or some other application controls this URI schemes are default handler, results are undefined and not certain. So, take proper countermeasures for those scenarios as well.