How to return access token as json result ?

See more:

MVC5

JWT

C#7.0

ASP.NET-Core

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 ?

What I have tried:

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);
        }