Inserts or updates an item in a collection.
The Save Data Item endpoint inserts or updates the specified item, depending on whether it already exists in the collection.
If you don't provide an ID, a new item is created.
If you provide an ID that does not exist in the collection, a new item is created with that ID.
If an item with the ID you provide already exists in the collection, that item is updated. When an item is updated, its data._updatedDate field is changed to the current date and time.
Note: When you provide an item with an ID that already exists in the collection, the payload you provide in dataItem.data replaces the existing item with that ID.
This means that the item's previous fields and values are lost.
You can only call this method when authenticated as a Wix app or Wix user identity.