Two solutions are available to you:
Using the login credentials of an Exchange Office 365 administrator
Create a new specific user for Signitic
Caution: In both cases Dual Authentication (2FA) must be disabled for the user you logged in to Signitic.
Log in with an administrator account to Outlook 365. Then go to the "Admin".
Create a new specific user for Signitic on the Outlook 365 control panel.
Fill in some basic information and enter do-admin as username.
Add a product licence to the user.
Add Exchange administration permissions to the user. This will allow the connection between Signitic and Office 365.
Finalize the creation of the user and go to the Signitic Office 365 connectors settings.
Enter credentials of the new Office 365 user in Signitic.