Step 1: Sign into https://portal.office.com/
Step 2: Navigate to Outlook web app or access https://outlook.office365.com/
Step 3: Click on the settings gear icon on the top-right corner of the page and select View all Outlook settings.
Step 4: Switch to Calendar > View tab and scroll down to Birthday calendar section.
Step 5: Check the Turn on birthday calendar checkbox and click Save.
Step 6: Now, go to OWA homepage and click on the Calendar at the bottom.
Step 7: Right click on My Calendars > Birthdays and select New birthday and enter the details and then click Create.