How to add javascript with sub-grid inside the entity

Am Using 2 entities in this process (contact & external).

Inside the contact entity( i displayed a Subgrid which showing  'external' entity records).

If i changed a field in 'contact' entity, it would create a new record in 'external' entity. I done the script.

Am trying to do in a reverse way, i.e., if someone create a new record using 'subgrid' then Add new record, the field should change back. If any idea/advise, please share

Thanks