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