I found I can retrieve it via REST API as soon as I do either one of following action;
1. I update the product without changing anything.
2. I put focus on the input field of “fb_product_item_id” and lose focus from that field(I don’t need to “Update”. Just focusing and unfocusing. I assume Ajax update is triggered).
I found that only accessing product’s admin edit page with browser is enough to be able to retrieve via REST API. I don’t even need to press Update button or focus/unfocus.
I don’t understand why accessing the admin product page triggers the ablility for API to retrieve “fb_product_item_id”.
