php - Get Product MPN (Manufacturer Part Number)

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Currently, I am working on an SEO app in Laravel. In this app, I am using JSON-LD for every product and I want to show every field recommended by Google's Rich Results Test Or Structured Data Testing Tool. I have imported my products to my store using CSV files.

The problem is that I want to show MPN number in structured data generated by every product But I can't find any documentation about the MPN number and I don't know how to retrieve this.

I have checked the products JSON but the MPN number field is not available there.

I found this URL and I followed all instructions But I can't fetch the MPN number

I also checked https://example.com/admin/products/product_id/metafields.json but not succeed.

--------UPDATE--------

In the official documentation of schema.org gives the details about the use of MPN or GTIN number in JSON-LD. I only want to find a way that gives me the MPN or GTIN numbers for any specific product.