g:mpn

I’m wondering how I use g:mpn instead of g:gtid. I created my feed and am getting the dreaded ‘Insufficient product identifiers: Missing two out of three attributes [GTIN, brand, mpn]’ message. I see in the google support docs that to use an mpn you should use g:mpn but I don’t see that field/option in the feed creation page.

Thanks-
June