i am in love with a guy . i went to a person as i was having a problem . he told me to forget the person as you wont be able to marry him ...he is not in your qadr alllah tallah says dua changes qadr ..so if make dua to get marrying that particular person with full faith and belief ... is it permissible ? and how can someone tell that a particular thing is not in your qadr when allah has promised that dua can change qadr
As for the fact that du'a can turn back the qadr or qada'a of Allah we have a couple of evidences:
And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided. (2:186 https://quranx.com/2.186)
which means that Allah answers a supplication. And in the sunnah:
"Nothing turns back the Decree except supplication, and nothing increases the life-span except righteousness." (Jami' at-Tirmidhi https://sunnah.com/urn/722840)
- “Nothing increases one’s life span except righteousness and nothing repels the Divine decree except supplication, and a man may be deprived of provision by a sin that he commits.’” (Sunan ibn Majah https://sunnah.com/ibnmajah/36/97)
Note that scholars say that du'a itself is from the qadr of Allah. So this doesn't mean that du'a can change what Allah decreed, but it can turn it back, as Allah decreed the du'a in first place too.
As for your statement that somebody told you that you won't be able to marry that guy, you either didn't share enough information with us (maybe this guy is your mahram or is already engaged and that person knows about it) or this person is a kind of charlatan as nobody is able to know what Allah has decreed for us.
As for making du'a it is rather recommended to ask Allah for guidance in general not in special as this shows more faith in what Allah has chosen for us and this was how the prophet used to make du'a. One exception certainly is istikharah where one should name the issue. But scholars allow to call things by name in a du'a too as quoted in the fatwa proposed by @servant-of-wiser in the comments: https://islamqa.info/en/answers/219260/is-it-permissible-for-a-man-to-pray-for-marriage-to-a-particular-woman-or-for-a-woman-to-pray-for-marriage-to-a-particular-man
Some fatwas in Arabic I've found useful: