Append to existing Description

I need help on this powershell command

Get-ADuser -searchbase "OU=TEST, OU=Stale Accounts, DC=com" | Set-ADuser $_ -Description "disabled due to inactivity, termrpt $($_.Description)"

What I'm trying to do is add more text to the description wihout deleting the existing description. 

When I run it, I get:

cmdlet Get-ADuser at command pipeline position 1

Supply values for the following parameters:

(Type !? for Help.)

Finter:

I've spend quite some time on this, but just can't get it to work.

Any help would be greatly apprciated.