populo-query-custom.cshtml

var results = Populo.Contacts.Query<ExplicitDataLog>(
                    new Query(
                        new[]
                        {
                            Op.EqualTo("trackingId", "MyTrackingId"),
                            Op.Exists("data.custom.myValue", true)
                        }));


Or:

var results = Populo.Contacts.Query<ExplicitDataLog>(
                    new Query(
                        new[]
                        {
                            Op.EqualTo("trackingId", "MyTrackingId"),
                            Op.Exists("myValue", true)
                        }));