Create a role for Outlook Web Access 365
If you authorize synchronisation with Outlook Web Access, you can create a Role with only 2 cmdlet activated:
Full access to get-mailbox and get-mailboxmessageconfiguration
Limited access to params Identity,AutoAddSignature and SignatureHtml for set-mailboxmessageconfiguration
You can upload this script with a detailed process of actions to do on Powershell:
https://sigilium.com/helpscout/scripts/office365_owa_role.ps1