No, it's not haraam
There are two hadiths about that... I'll quote you both
"It was narrated from ‘Umar, Ibn ‘Umar and Zayd ibn Thaabit (may Allaah be pleased with them) that it is allowed to urinate standing up on the condition that there is no risk of drops of urine splashing onto one’s body or clothes, and that one does not expose one's ‘awrah, because al-Bukhaari and Muslim narrated from Hudhayfah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) came to a garden belonging to some people, and urinated standing up. There is no contradiction between this and the hadeeth of ‘Aa’ishah
The one of Aisha(MABPWH)
It is not haraam for a man to urinate standing up, but it is Sunnah for him to urinate sitting down, because ‘Aa’ishah (may Allaah be pleased with her) said: “Whoever tells you that the Prophet (peace and blessings of Allaah be upon him) used to urinate standing up, do not believe him. He only ever used to urinate sitting down.” (Narrated by al-Tirmidhi, al-Tahaarah)
You probably misunderstood this as the prophet used to pee standing up sometime
It's not haraam to pee standing up, it's just that it's advised peeing while sitting down as it's sunnah.
If you know nothing about something, it's better to be quiet especially when it's about the deen.