The uri is the fully qualified domain name of the exchange server, so if your exchange server was 'srv-exchange' and your domain was 'college.local' then the fqdn would be 'srv-exchange.college.local'
You can sometimes avoid the + thing totally, powershell does let you use vars in strings like this:
$FirstName = "Bill"
$LastName = "Hats"
"$FirstName.$LastName"
"$FirstName, $LastName"