1. Sign into your Office365 administrator account.
2. Click the App Launcher icon in the top left-hand side.
3. Select Admin.
4. Expand the Users menu on the left-hand side and select Active Users.
5. Click Add a User.
6. Enter the required information into the form. In the Password section, check 'Let me create the password' and uncheck 'Make this user change their password when they first sign in'.
7. In the Roles section, select 'Customized Administrator', and check 'Exchange administrator' and 'Service administrator'.
8. Finally, in the Product licences section, make sure you select 'Create user without product license'.
9. Now you need to assign the Exchange permissions to your service account. To do this expand the Admin centers menu and select Exchange.
10. In the window that opens you will see the Exchange admin center dashboard. Now click on Permissions.
11. Click on the plus icon to add a new role group.
12. Give the role group a name. Click on the plus icon by Roles and add the ApplicationImpersonation role. Then click on the plus icon by Members and add the user account you created earlier.
13. Click Save.
Learn how to use your Service Account to connect and manage multiple mailboxes by reading the following article: