You can request an eval key via MyNetworking Portal (https://www.hpe.com/networking/mynetworking), or contact your local Aruba partner or Aruba SE.
When registered and login go to register license and select a ClearPass 90-day eval (Lic-1-ClearPass). That should run for 90 days, and you can replace the key and subscription id when you have your production license.
Make sure you select the CP-VA-5K when deploying your VM.