UserStuff.cs

    internal class User: IUser
    {

        async Task<TLoginUserResponse> IUser.LoginUser<TLoginUserDTO, TLoginUserResponse>(TLoginUserDTO loginUserDTO)
        {

    ...
            var loginUserDTODerived = (LoginUserDTO) (ILoginUserDTO) loginUserDTO;
            var dummy = loginUserDTODerived.NewProp; 
    ...
            return (TLoginUserResponse) (ILoginUserResponse) new LoginUserResponse
                {NewProp = someValue, NewProp2 = someValue2};
        }
    }

    public interface IUser
    {
        Task<TLoginUserResponse> LoginUser<TLoginUserDTO, TLoginUserResponse>(TLoginUserDTO loginUserDTO)
            where TLoginUserDTO : ILoginUserDTO
            where TLoginUserResponse : ILoginUserResponse;
    }

    usage:

        var getUserResponse = await user.LoginUser<LoginUserDTO /* having some NewProp */, LoginUserResponse /* having some NewProp */>(new LoginUserDTO
        {
            DefaultProp1 = someValue, DefaultProp2 = someValue2, NewProp = someValue3
        });