Get-PartnerMailboxes.ps1

Function Get-PartnerMailboxes{
    
    [cmdletbinding()]

    Param(
        [Parameter(Mandatory=$True)]
        [PSCredential]$Credential = (Get-Credential),

        [Parameter(Mandatory=$True)]
        [String]$Path
    )

    Import-Module MSOnline
    Connect-MsolService -Credential $Credential
    
    $clients = (Get-MSOLPartnerContract)
    $tenants = $clients.tenantid

        ForEach($Tenant in $Tenants){

        $Users = Get-MsolUser -TenantId $Tenant| Where-Object {$_.IsLicensed -eq $True}
        $UPN = $Users[0].UserPrincipalName
        $Domain = $UPN.Split('@')

        $Properties = @{'Licensed Users' = $Users.Count;
                        'Domain' = $Domain[1];
                        }  
    
        $PropsObject = New-Object -TypeName PSObject -Property $Properties
        $PropsObject | Export-CSV -Path $Path -NoTypeInformation -Append  
        }
}

Get-PartnerMailboxes -Path C:\Users\Jacob\Desktop\PartnerMailboxes.csv