How to return access token as json result ?

Question

Problem

How to return Access Token as json result ?

I work on asp.net core 2.2 from function below i generate access token

it is success generated as string but i need to generated as json

How to generate access token as json from function below ?

public string GenerateTokens(string userId)
{
    var Claims = new Claim[]
    {
        new Claim(JwtRegisteredClaimNames.Sub,userId)
    };
    var signingkey = new
    SymmetricSecurityKey(Encoding.UTF8.GetBytes("this is secret phrase"));
    var SigningCredntials = new SigningCredentials(signingkey,
    SecurityAlgorithms.HmacSha256);
    var Jwt = new JwtSecurityToken();
    var jsonu = new { id = userId };
    Jwt.Payload["user"] = jsonu;
    return new JwtSecurityTokenHandler().WriteToken(Jwt);
}