Set Edm.ComplexType in Azure Search by SDK

I have a problem in Azure search, I get data from a csv file in blob storage. To simplify let assume my object as:

public class Instrument
{
public Identifier Identifier { get; set; }

[SearchableField(IsSortable = true, IsKey = true)]
public string Id { get; set; }

[SearchableField(IsSortable = true)]
public string RefIs { get; set; }

}

public class Identifier
{
[SearchableField(IsSortable = true)]
public string Code { get; set; }

}
when I receive my data it's flat in the csv such as : _id,_ref,_code.

When I create my index through SDK I setup Mapping for simple Fields:

    indexer.FieldMappings.Add(new FieldMapping("_ref")
    {
        TargetFieldName = "RefIs"
    });

but I cannot figure out the way I declare it to complex type in my indexer ? given code does not work for it :

    indexer.FieldMappings.Add(new FieldMapping("_code")
    {
        TargetFieldName = "Code"  
    });

    indexer.FieldMappings.Add(new FieldMapping("_code")
    {
        TargetFieldName = "Identifier.Code"  
    });

error is the same TargetField is not present in index.

Could someone help ?