Disallow Subscription change in Azure Service catalog managed application definition

Question

How to disable the subscription dropdown input in my service catalog managed app definition, for ex:

{
  "$schema": "http://schema.management.azure.com/schemas/0.1.2-preview/createUIDefinition.MultiVm.json#",
  "handler": "Microsoft.Azure.CreateUIDef",
  "version": "0.1.2-preview",
  "parameters": {
    "config": {
      "isWizard": false
    },
    "basics": [
      {
        "name": "subscription",
        "allowedValues": [
          "[subscription()]"
        ]
      },
      {
        "name": "vmName",
        "label": "Virtual Machine Name",
        "tooltip": "Name of the virtual machine",
        "type": "Microsoft.Common.TextBox",
        "defaultValue": "myVM",
        "constraints": {
          "required": true,
          "regex": "^[a-zA-Z][a-zA-Z0-9-]{1,61}[a-zA-Z0-9]$",
          "validationMessage": "The name must begin with a letter and contain only letters, numbers, and hyphens. The name must end with a letter or number."
        }
      }
    ],
    "steps": [],
    "outputs": {
      "location": "[location()]",
        "vmName": "[variables('vmName')]"
    }
  }
}