Step 1: Sign into Office 365 admin portal
Step 2: On the left pane, click on Users >> Active Users
Step 3: Click on an account and scroll down to Multifactor authentication area
Step 4: Click on Manage multifactor authentication
Step 5: On the new Active Directory window click on the account to which you would like to enable Multi-factor authentication
Step 6: From the right pane, click on Enable