Install-Module -Name ExchangeOnlineManagement
Connect-ExchangeOnline
3. Once signed-in use the command below to select and convert the desired account to Shared:
Set-Mailbox yourmailboxusername@yourdomain.com -Type shared
4. Verify the mailbox has Shared status.
Get-Mailbox -Identity yourmailboxusername@yourdomain.com | Format-List RecipientTypeDetails