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Since it's Frage nach one would think that it would be Anfrage nach, but I have to agree with the article. Can't tell you if it's completely incorrect, since historically, it wouldn't have been used this way in the first place. (It would have been something like anfragen bei jemandem + relative clause)

Personally, I wouldn't recommend nach, because (following my intuition) the two prepositions, an and nach, don't fit together, here.

So Anfrage in connection with some noun usually comes with wegen:

Anfrage wegen dem Angebot
Anfrage wegen des Angebots
Wegen dem Angebot anfragen
Wegen des Angebots anfragen

Note however, that this is not asking for some offer, but about it. Also consider simply using Anfrage, if it's clear from context what the Anfrage is about.