Convert Lead - Related Field - initial_filter question

I have added a related field to the converted lead page. It is related to the users module. The related field works as it should. But now I have a requirement to filter on the title. I have tried the code below but it is now working.

array (
                array ('name' => 'assigned_user_name', 'label' => 'LBL_ASSIGNED_TO'),  
                'displayParams' => array ('initial_filter' => '&title_advanced=Administrator'),

Can someone point me in the right direction?