Hi,
Unfortunately I don’t know the custom field name, but there is a very effective way to find it.
Go to the Search Sources -> Post Type Search panel, and scroll down to the “..or search in selected custom fields” option: https://i.imgur.com/t6TWfiD.png
There is a search input there, to look for existing custom fields. Try search for “rank”, “rankmath” and you will get a list of custom fields matching that name – and then you can use those in the advacned fields 🙂
