Migration serial object with a parent object

Hello Data Migration Wizards,

I am trying to migrate several “child” serial objects. The ‘parent’ objects already exist. I am using FNDMIG migration jobs for this. This allows me to create the serial objects. However, the field “Belongs_To_Object aka (SUP_MCH_CODE)” always remains empty. As a result, I cannot create a parent-child relationship. 

If I enter the “Parent Object” manually, it works!

 

I understand that the field is only insertable, meaning it can't be updated. However, I can't insert new data or update it. I don't get an error message either. The field is simply left blank.

I've tried various things, including entering the equipment sequence. Nothing has worked. So, how should I populate the field using a migration job?

 

​@Marcel.Ausan 

Thanks in advance.