I don't think you can.
You can add userName and password as options to WKHTMLtoPDF but then you need everybodys passwords which makes that worthless.
But there is a workaround. You can use windows impersonation in IIS and therefore bypass this issue.
See this link on msdn to configure it.
Edit
You need to activate NTML v1 on server, see Cutter1703 comment below.