using System.Runtime.Serialization;

namespace Test
{
    [DataContract]
    public class LoginRequest
    {
        [DataMember(Name = "username")]
        public string UserName { get; set; }
        [DataMember(Name = "password")]
        public string Password { get; set; }
    }
}

using Newtonsoft.Json;

namespace Test
{
    public class LoginRequest
    {
        [JsonProperty("username")]
        public string UserName { get; set; }
        [JsonProperty("password")]
        public string Password { get; set; }
    }
}