FIM Powershell Password Reset Workflow

Hello All,

I'm trying to use the FIM portal in order to allow helpdesk to reset an users password upon the end user calling helpdesk. But am running into an issue with the powershell workflow.


Currently I have added a boolean attribute called "PasswdReset" and binded it to the user type.
Created an criteria based set called "PasswordResetUsers" which will transition users into when passwdrest is set to true.
Created an workflow called "PasswordResetWF" which is made up of several steps
1. The powershell activity which executes the following script

Import-Module Activedirectory
$newpwd = ConvertTo-SecureString "P@ssw0rd" -AsPlainText –Force
Set-ADAccountPassword SampleUser -Credential domain\administrator -NewPassword $newpwd -Reset -PassThru | Set-ADuser -ChangePasswordAtLogon $True

Here lies the problems
First how do I pass the samaccountname of the user to the powershell script to replace the sampleuser parameter.
Second How do I  force this script to run as the domain\administrator without prompting for password

The rest of the workflow is as follows.
2. Trigger custom email notification to an monitored email account for logging purpose
3. Trigger custom email notification to Helpdesk monitored email account to generate support ticket. 
4. OOB function to set "PasswdReset" attribute to false and transition the user out of the "PasswordResetUsers" set.

As always any suggestions would be greatly appreciated.