Microsoft 365 – Adding user access to mailbox via PowerShell

This will enable you to open shared mailboxes using the Open Another Mailbox option in Outlook Web App

1. Run PowerShell as Admin
2. Connect to Exchange Online
   1. Install-Module -Name ExchangeOnlineManagement -Force
   2. Connect-MsolService
   3. Connect-ExchangeOnline -UserPrincipalName [admin email address] -ShowProgress $true
3. Add-MailboxPermission -Identity [user mailbox to share] -User ‘[user account]‘ -AccessRights FullAccess -InheritanceType All -AutoMapping $true