Is it possible to know the result of Salatul-Istikhara before performing it? And is it obligatory to perform this prayer to ascertain the answer, given that a dream about a young man was seen prior to performing it?

Istikhara prayer is recommended, not obligatory, if a person intends to do something permissible. It is sunnah for a servant to seek guidance from his Lord and consult those with sound judgment and experience, and then proceed with the matter. If his heart is at ease and Allah makes it easy for him, then it is good. Otherwise, it is not good. Istikhara does not necessarily entail a dream or anything else. If there is nothing that necessitates Istikhara, then it should not be performed. If the young man proposes to you, then it is permissible for you to perform Istikhara and seek consultation. You must cut off any unlawful relationship with him.