Why do I get Parse error (code 400) from Google Ftiness API in Unity?

I am trying to read steps count from GoogleFitness API in Unity application. First I go to Google's OAuthPlayground, authorize the fitness api and copy the access token. In Unity I have a scene with text field and a button. I paste the access token to the textfield and click the button. On Button click the method GetStepsResponse is being called with input field's content as a param.

The logic behind:

     public void GetStepsResponse(string idToken)
 {
 
     StartCoroutine(SendPostCoroutine(idToken));
 }
 
 IEnumerator SendPostCoroutine(string accessToken)
 {
     string request = "https://www.googleapis.com/fitness/v1/users/me/dataset:aggregate";
     request = string.Concat(request, "?access_token=", accessToken);
     string body = "{  \"aggregateBy\": [{    \"dataTypeName\": \"com.google.step_count.delta\",    \"dataSourceId\": \"derived:com.google.step_count.delta:com.google.android.gms:estimated_steps\"  }],  \"bucketByTime\": { \"durationMillis\": 86400000 }, // This is 24 hours  \"startTimeMillis\": 1577059200000, //start time  \"endTimeMillis\": 1577577600000// End Time}";
 
     UnityWebRequest webRequest = UnityWebRequest.Post(request, body);
     webRequest.SetRequestHeader("Content-type", "application/json");
 
     yield return webRequest.SendWebRequest();
 
     Debug.Log(webRequest.downloadHandler.text);
 }

as a response I get:

 {
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "parseError",
    "message": "Parse Error"
   }
  ],
  "code": 400,
  "message": "Parse Error"
 }
 }

I tried the same request with same body and headers here: https://apitester.com/ and I got a proper response with steps count for every day from given range. What I am missing?

EDIT

I came up with an idea that escaping the quatiion mark in the request body may cause the Parse Error, so I switched from manually constructing a request body string to serializing an object to JSON and using the product string as a body:

 public void GetStepsResponse(string idToken)
     {
 
         StartCoroutine(SendPostCoroutine(idToken));
     }
     IEnumerator SendPostCoroutine(string accessToken)
     {
         string request = "https://www.googleapis.com/fitness/v1/users/me/dataset:aggregate";
         request = string.Concat(request, "?access_token=", accessToken);
 
         FitnessRequest requestBody = new FitnessRequest();
         requestBody.aggregateBy = new AggregateByRequestParam[1];
         requestBody.aggregateBy[0] = new AggregateByRequestParam();
         requestBody.aggregateBy[0].dataTypeName = "com.google.step_count.delta";
         requestBody.aggregateBy[0].dataSourceId = "derived:com.google.step_count.delta:com.google.android.gms:estimated_steps";
         requestBody.bucketByTime = new DurationRequestParam();
         requestBody.bucketByTime.durationMillis = 86400000;
         requestBody.startTimeMillis = 1577059200000;
         requestBody.endTimeMillis = 1577577600000;
 
         string body = JsonUtility.ToJson(requestBody);
         Debug.Log(body);
 
         UnityWebRequest webRequest = UnityWebRequest.Post(request, body);
         webRequest.SetRequestHeader("Content-type", "application/json");
 
         yield return webRequest.SendWebRequest();
 
         Debug.Log(webRequest.downloadHandler.text);
     }
 
     [System.Serializable]
     private class FitnessRequest
     {
         public AggregateByRequestParam[] aggregateBy;
         public DurationRequestParam bucketByTime;
         public long startTimeMillis;
         public long endTimeMillis;
     }
 
     [System.Serializable]
     private class AggregateByRequestParam
     {
         public string dataTypeName;
         public string dataSourceId;
     }
 
     [System.Serializable]
     private class DurationRequestParam
     {
         public long durationMillis;
     }

However the result is still the same. Again, when I use the same request url and request body outside Unity it works (I get code 200 as a response). Is there something specific in Unity's UnityWebRequest.Post that I'm missing?