Start service using windows domain account

Hi i am trying to write a powershell script which starts ReportServer service using a windows domain account. Below is what i have got so far:

$password = cat C:\securestring.txt | convertto-securestring
$cred = new-object System.Management.Automation.PSCredential $username,$password

Start-Service "SQL Server Reporting Services (MSSQLSERVER)" -credential $cred

Everytime i run this i keep getting the error:

Start-Service : A parameter cannot be found that matches parameter name 'credential'.
At line:12 char:72
+ Start-Service "SQL Server Reporting Services (MSSQLSERVER)" -credential <<<<  $cred
    + CategoryInfo          : InvalidArgument: (:) [Start-Service], ParameterBindingException
    + FullyQualifiedErrorId : NamedParameterNotFound,Microsoft.PowerShell.Commands.StartServiceCommand

I cannot figure out what i am doing wrong, any help is much appreciated. Am i putting the -credential argument in the wrong place?