Right syntax for fieldName value in Cloud Firestore module❓

Hi all,
I’m getting : Missing value of required parameter ‘fieldName’
My schema to update: giftList [{"cs": "some value"}, {"status": "pending"}]
Trying to figure out the right syntax for the field name ‘cs’ which is in a specific object [index ‘ix’] inside array giftList. What is the proper way to use the formula for getting to the fieldName of the desired object?