Please review the scriptable task, inputs, and bindings of the attached workflow that I created when originally answering your question...
You may import the attached workflow and test/modify it as desired.
Workflow Inputs:
computerAD (AD:ComputerAD) -- this is the computer object in the AD plug-in that will have its description field modified
newDescription (string) -- the new value of the description for the selected computer object
Scriptable task:
The scriptable task input has both of the workflow Inputs bound to it allowing access to these values within the script.