Add and update a list of things

Hi, the data structure is as such.
1 User - main user
Multiple customers, where one of the field is parcel ID
A customer can have many different shipment / parcel id

How then can I add a new parcel to the list of things under a customer when I create a new shipment?

Even though the customer ID under parcel is a text field, which it should be, the selection is taken from a dropdown box where it grabs existing customers as data.

But I am stuck in trying to update my parcel ID list under my customer ID