Explanation

Code to report fraud during initial greeting: When users receive a phone call to perform two-step verification, they normally press # to confirm their sign-in. To report fraud, the user enters a code before pressing #. This code is 0 by default, but you can customize it.
Block user when fraud is reported: If a user reports fraud, their account is blocked for 90 days or until an administrator unblocks their account. An administrator can review sign-ins by using the sign-in report, and take appropriate action to prevent future fraud. An administrator can then unblock the user's account.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/authentication/howto-mfa-mfasettings#fraud-alert

Reference:
https://docs.microsoft.com/en-us/deployoffice/configuration-options-for-the-office-2016-deployment-tool

Explanation
* Vendors must be able to authenticate by using their Microsoft account when accessing Contoso resources.
You can invite guest users to the directory, to a group, or to an application. After you invite a user through any of these methods, the invited user's account is added to Azure Active Directory (Azure AD), with a user type of Guest. The guest user must then redeem their invitation to access resources. An invitation of a user does not expire.
The invitation will include a link to create a Microsoft account. The user can then authenticate using their Microsoft account. In this question, the vendors already have Microsoft accounts so they can authenticate using them.
In this solution, we are creating guest account invitations by using theNew-AzureADMSInvitationcmdlet and specifying the-InvitedUserEmailAddressparameter.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/b2b/add-users-administrator
https://docs.microsoft.com/en-us/powershell/module/azuread/new-azureadmsinvitation?view=azureadps-2.0