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.

*Bezüglich* is a nice choice, too (comes with genitive, see other answer).