Using facebook custom_number_0 to create product set

Aloha friends!

I’d like to create product sets made of specific variants I selected. I am unable to specify exactly which variant I want to add to my product set.

I’d love to be able to tag the variants in shopify and and create a product set in facebook that is conditionally defined on that tag.

I’ve tried adding a metafield to the variants called facebook-ad, I am not sure how often shopify propagates changes to facebook so I am just waiting.

But it also seems like facebook may have more explicit structure and Ill need to use one of the tags described here: https://business.facebook.com/business/help/120325381656392?id=725943027795860

I think custom_number_0 would work for me (or custom_label_0). I am just not sure how to set it on a variant and propagate that to facebook. Does anybody have experience sending additional fields to facebook?

Thank you!

Boaz