Extending VM Harddisk using $extendedCapacity

Greetings All,

I am trying to extend the OS partition after I use the Set-harddisk command in this format. Can anyone assist on how the syntax would be written please?

get-harddisk -vm testvm | where {$_.Name -eq "Hard disk 1"} | set-harddisk -CapacityKB 39845888

How / Where would I input this command?

( $extendedCapacity -HostCredential $hostCred -GuestCredential $guestCred) -

*testvm is my vm hostname

Thank you so much!