For the multifactor authentication a new app in Entra ID is necessary. You can also use your dox42 Online app. The app will be mapped inside D365 FSCM to a specific (service) user and claim its roles and rights.
Steps
Create a new app registration and client secret like in the dox42 Online Setup.
These will be your sign-in credentials in the dox42 Word Add-in.Certificates & secrets page
In D365 FSCM go to System Administration > Setup > Azure Active Directory applications.
Fill in your Client ID, give it a name optionally and map it to a specific (service) user inside D365 FSCM.
Azure Active Directory applications page
Result
A connection Excel file should then be created. For more details on this please see Creating the connection Excel file.
Use those newly created credentials to connect to your D365 FSCM instance, by either filling them in directly, or - the more recommended approach – by using your created connection Excel file.

D365 FSCM Report Data Source window