The shariah commands us to urinate sitting down. The Prophet’s (peace be upon him) habit was to urinate sitting down as well.
Hazrat Umar Farooq (may Allah be pleased with him) said that the Prophet Muhammad (peace be upon him) once saw him urinating standing up, and the Prophet (peace be upon him) forbade him and said, "O Umar! Do not urinate standing up." Hazrat Umar Farooq (may Allah be pleased with him) said: “I never urinated standing up after that.” (Sunan Tirmidhi, Hadith No. 12)
Urinating standing up without a legitimate excuse is deemed makrooh and should be avoided because it requires the exposure of the private parts which leads to immodesty, and involves the risk of urine droplets splashing onto the body. Additionally, it is an imitation of non-Muslims, which further highlights the unappealing nature of this act.
That being said, urinating standing up is excused in cases where there is a legitimate compulsion or excuse to do so.