EncryptingJsonConvertor in WebAPI

Question

I tried using the solution of encrypting some properties of my class in JSON mentioned by Thomas Freudenberg. When I serialize the object using JsonConvert.SerializeObject() I get the desired result.

The class and convertor are located in a separate assembly, which is referenced in my WebAPI project. When I use the following:

[HttpPost]
[Route("api/ApiUsers/List")]
public Dto.ApiUsersResponse List(Dto.ApiUsersRequest apiUsersRequest)
{
    IEnumerable<Db.ApiUser> dbApiUsers = GetList<Db.ApiUser>(StoredProcedures.GetApiUsers);

    return new Dto.ApiUsersResponse()
    {
         ApiUsers = dbApiUsers.Select(apiUser => new Dto.ApiUser()
         {
            ClientId = apiUser.ClientId,
            ClientSecret = apiUser.ClientSecret
         })
    };
}

the result is in clear text instead. The conversion never happens.

I have tried changing the controller's response IHttpActionResult and using Ok() and Content() but to no avail.

When I try to register the convertor using:

config.Formatters.JsonFormatter.SerializerSettings.Converters.Add(new EncryptingJsonConverter());

The error is:

Cannot resolve symbol 'JsonFormatter'

What am I missing?