{
   "if": 
   {
      "allOf": [
         {
            "field": "[concat('tags[', parameters('BackupPolicyTagName'), ']')]",
            "exists": "false"
         },
         {
            "field": "type",
            "equals": "Microsoft.Compute/virtualMachines"
         }
      ]            
   },
   "then": 
   {
      "effect": "append",
      "details": [
      {
         "field": "[concat('tags[', parameters('BackupPolicyTagName'), ']')]",
         "value": "[parameters('DefaultTagValue')]"
      }
      ]
   }
}