Is it permissible to supplicate to Allah to marry a specific person, and is it valid to perform Istikhara (prayer for guidance) after such a supplication?

There is no harm in supplicating for marriage to a specific person, for supplication is legislated and does not contradict the belief that marriage and other matters of people's lives are predetermined and written fifty thousand years before the creation of the heavens and the earth. It is also commendable to perform Istikhara regarding accepting this man, while entrusting the matter to Allah Almighty and emptying the heart of all personal thoughts and inclinations.