$ACL = Get-ACL .\Temp\
$Rule = [System.Security.AccessControl.FileSystemAccessRule]::new(@"computername\user",[System.Security.AccessControl.FileSystemRights]::FullControl,[System.Security.AccessControl.AccessControlType]::Allow)
$ACL.Access.AddRule($Rule)
Set-Acl -Path .\Temp\ -AclObject $ACL