Getting 403 Forbidden error for fetching message from a shared mailbox using Graph API with get an open extension object identified by name.

Question

I want to get a message from a shared mailbox using Graph API for an Add-in for Outlook for Mac with get an open extension object identified by name. I can access this for a personal mailbox( /v1.0/me/messages/<itemId>?$expand=Extensions($filter=Id eq <customPropertyId>) ) without any issues but when I try to do the same for a shared mailbox I am getting 403 Forbidden. (/v1.0/users/<sharedMailbox ID>/messages/<itemId>?$expand=Extensions($filter=Id eq <customPropertyId>)). I have given the application the following permissions. Can someone let me know if this is possible for a shared mailbox and if so what other permissions need to be given for the same.