If your organization is using multi-factor authentication (MFA) for Microsoft 365, the easiest verification method to use is the Microsoft Authenticator smartphone app.
Next, set up Microsoft Authenticator for your account:
Select Microsoft Authenticator as your method for authentication
Click Set it up now, choose Mobile app from the dropdown, and make sure "Receive notifications for verifications" is selected. Click Set up.
You should see a window on your computer that looks like this:
Add your account to Microsoft Authenticator:
Open the Microsoft Authenticator app on your phone. Tap the + > Work or school account. Use your phone to scan the QR square that is on your computer screen.
[Note: iPhone users may have to enable the camera in Settings in order to scan. If you can't use your phone camera, you'll have to manually enter the 9 digit code and the URL.]
Your account will be added automatically to the app and will display a six-digit code.
Confirm activation status on your computer
Switch back to your computer and click Done. Now, wait for the Checking activation status text to finish configuring your phone. When it's complete, you'll be able to click the Contact me button on the right.
Approve sign-in on your phone
Switch back to your phone and you'll see a notification for a new sign-in. Go to the Microsoft Authenticator app. Tap Approve to allow it.
Finish the setup process
Back on the computer, follow any prompts that you might see such as adding a mobile number.
From now on, whenever you have a new sign-in or add your Microsoft 365 work or school account to an app, you'll open the Authenticator app on your phone and tap Approve.