This file contains the parameters for the template and is called from within the preceding PowerShell script. These parameters will overwrite the parameters in the Template file.  You can use multiple parameter files to deploy to different environments; I usually also create a different PowerShell file as well by environment so I do not need modify it. I add the parameters for username and password, as well as the private Subnet definition:

{
  "$schema": "http://schema.management.azure.com/schemas/2015-01-01/deploymentParameters.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "adminUsername": {
      "value": "azureuser"
    },
    "adminPassword": {
      "value": "Azure12345678"
    },
    "PrivateSubnet": {
      "value": "10.0.1.0/24"
    }
  }
}