Use Select-Object's -ExpandProperty parameter, instead of -Property (which is what you are using when you pass arguments positionally):
$managername = get-aduser $listmanager | select-object -ExpandProperty name
The difference is that $managername will now be a String object, instead of a PSObject that has a String property called "Name". When you see that hashtable-like syntax in a string, it's a dead giveaway: "@{name=Manager Name}"