First.Last to DN Workflow

We've got Okta pushing manager first.last to extensionAttribute3 in ARS. I need a workflow that will convert that to a DN to drop into the manager field. However, no matter what I do, it doesn't want to get from any attributes.

Workflow setup:

On Demand
EA3 Search - Filters from an OU with 4 users. No other filters in place. --> Run Script: First.Last to DN

First.Last to DN script

function onPostModify($Request)
{
$sourceObj = $workflow.FoundObject("EA3").get("DN")
$manager = get-aduser -identity $sourceObj -properties extensionattribute3 | select -expandproperty extensionattribute3
$managerDN = (get-aduser -id $manager).distinguishedname	
$dirobj.put("manager", $managerDN)
$DirObj.SetInfo()
}

The error I get when I try to run the workflow with the script is: