Unfortunately there is not a good answer to this question (as far as I know). Webflow does not allow you to access CMS items ids with integrations like Zapier. The most straight forward solution would be to create a “Item ID” field in the CMS item and number all of your items. Or just use the items url slug as an ID since you know that will be unique for each CMS item.
I haven’t personally set this workflow up, but if you’re accessing Collection items then it seems you will need to provide the correct field so Zapier can fetch it nad modify it.
In the object the key value pair that Zapier accesses is "_id": "idNumberHere" in this case the key that Zapier would likely need is _id. I wonder if just using id would be causing issues?