[HttpPost]

    public async Task<ActionResult<int>> Process()
    {
        string jsonString = await GetRequestBody();
         ...
    }

    private async Task<string> GetRequestBody()
    {
        string jsonString;
        using (StreamReader reader = new StreamReader(Request.Body, Encoding.UTF8))
        {
            jsonString = await reader.ReadToEndAsync();
        }

        return jsonString;
    }