Updating a linked record field in Airtable without overwriting

I am updating a membership database to link my customers table to an event table.

In Zapier I could grab the field data from a previous step to put in back in with the new identifier linked. So it looks like this: "[Mapped data from previous recrord],newrecordID"

Here each existing linked record is a separate item and I need them all together separated by commas. Is there any easy way to generate this?
 
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