how to authenticate o365 rest api without login (by giving username and password from backend)

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MVC5

In my project I have to update o365 calendar for client. I have a table in which session for a calendar is saved which we use internally. When user clicks on a calendar button data from table will insert in o365 calendar.

Problem is we don't want give o365 default login to end user. We have our own login system. Once user get login in our system he doesn't need to login for anything again.