Answer
(Fatwa: 54/45/N=1433)
(1) Yes, it is allowed to supplicate Allah with the wasilah of the holy Prophet Muhammad (صلى الله عليه وسلم) for oneself, for one’s parents etc for forgiveness or whatsoever. Rather it is mustahab to supplicate there, because it is a place where supplications are accepted. The scholars of Fiqh have mentioned it in the etiquettes of ziyarah.
(2) One may request the holy Prophet Muhammad (صلى الله عليه وسلم) for dua at his holy grave, rather it is also mustahab. The scholars have mentioned it as well. But one cannot request the holy Prophet Muhammad (صلى الله عليه وسلم) for dua while he is away from the holy grave, because the holy Prophet Muhammad (صلى الله عليه وسلم) is not omnipresent, it is only Almighty Allah who is Omnipresent.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband