Writing Tests for .NET API

I am very new to .NET / C#.

I am using jwt for means of authentication. I have this object:

var token = new JwtSecurityToken(
  issuer: "example.com",
  audience: "example.com",
  claims: claims,
  expires: DateTime.Now.AddDays(TOKEN_EXPIRY_DAYS),
  signingCredentials: creds
);

However, I manually add a "created" property:

token.Payload["created"] = DateTimeOffset.Now.ToUnixTimeSeconds();

All I need to do is to write a test that checks to see if the token object has an attribute called "created".

I have tried something like this:

tokenResponse = JsonConvert.DeserializeObject<TokenResponse>(
  await response.Content.ReadAsStringAsync()
);
Assert.NotEmpty(tokenResponse.token["created"]);

But this fails. Any ideas?