Accessing a shared office365 mailbox via powershell - powershell

Question

I have shared mailbox on office365 which doesnt have a user account associated with it i.e. I cant login to it via the office 365 portal. I access to it via Outlook as a shared mailbox hence my account has permission to view the inbox.
I have used the powershell script found here and I have barely amended it except to change the $mail and $password.
If I use my own account credentials I can loop through the foreach loop and observe it accessing my emails which is great. I want to however access a shared mailbox called shared#mailbox.com instead of myAccount#mailbox.com.
To attempt to do this I added a username variable and passed this to $service.credentials and passed the $mail variable to $service.autodiscover. See code below.
This did not work....
Can anyone help?
$username="myAccount#mailbox.com"
$password="myPassword"
$mail = "shared#mailbox.com"
# Set the path to your copy of EWS Managed API
$dllpath = "C:\Program Files\Microsoft\Exchange\Web Services\2.2\Microsoft.Exchange.WebServices.dll"
# Load the Assemply
[void][Reflection.Assembly]::LoadFile($dllpath)
# Create a new Exchange service object
$service = new-object Microsoft.Exchange.WebServices.Data.ExchangeService
#These are your O365 credentials
$Service.Credentials = New-Object Microsoft.Exchange.WebServices.Data.WebCredentials($username,$password)
# this TestUrlCallback is purely a security check
$TestUrlCallback = {
param ([string] $url)
if ($url -eq "https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml") {$true} else {$false}
}
# Autodiscover using the mail address set above
$service.AutodiscoverUrl($mail,$TestUrlCallback)