public IHttpActionResult Post(TestViewModel value)
{
     //Código
}

public class TestViewModel
{
   public IDictionary<string, int> Ids { get; set; }
}

public Dictionary<string, int> Ids { get; set; }

Ids.Add("Llave1", 1);
Ids.Add("Llave2", 2);
Ids.Add("Llave3", 3);