I manually entered a storage account name ("jenkinsvmagents_storage") and clicked "Verify Template" which worked properly and said that the template was verified.

However upon the deployment of an agent, there is an ARM Deployment failure in the portal:

 "error": {
    "code": "AccountNameInvalid",
    "message": "jenkinsvmagents_storage is not a valid storage account name. Storage account name must be between 3 and 24 characters in length and use numbers and lower-case letters only."
  }