Item custom child record type

Hi;

I am trying to add a custom record and attach it to items file as a child record. I added a field in the custom record, type - List/Record, List/Record - Item. Apparently with a definition like that, the data expected in the field includes the whole path to parent items, such as "parent-a:Parend-B:ItemID". I need the field to contain only the itemID (this is how it comes from a datafeed). So I can define the field as free text instead, but I still want to have it as a child record to item. So how can I create the