powershell script for assigning \changing ownership of a shared mailbox

Can someone please tell me shell script for assigning or changing ownership to a mailbox ?!

I am new to powershell, I have to update ownership of 200 shared mailboxes and would really prefer to run a script rather than use AD or EMC ..

I found this :

Add-MailboxPermission -Identity "mailboxName" -Owner " Jonh Doe"

It runs but the output remains unchanged  

here's the parameter that I need updated:

What am i missing here?