Mark Vaillancourt explains Azure automation credentials:
With that String parameters, it is easy to just type what I want in the textbox. But, for the PSCredential parameter at the bottom, I was unsure what to do. I figured, since this is just a textbox, I couldn’t just magically pass in a Credential Asset. So, I tried passing in an expression that used the Get-AzureRmAutomationCredential cmdlet that returns the Credential Asset specified. That didn’t work very well. I then started digging through documentation, figuring I would see references and examples of passing a Credential Asset in the Test Pane. It took me a bit, but I finally landed on an Azure.com blog post titled Azure Automation: Runbook Input, Output, and Nested Runbooks. While this post didn’t match all that closely with the search criteria around the Test Pane, it did contain my answer in a section on starting Runbooks:
The answer turns out to be pretty simple.