I can't control my urine.After the time of prayer starts I pour water on my underwear and make wudu.I read in your fatwas that it is obligatory to put a cloth on penis.I don’t put any cloth on my penis because it keeps moving from its place and touches my pant.Sometimes I don't wear underwear. Is my salat valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We hope that your prayer will be valid. Some jurists are of the view that it is desirable for the person who suffers from urine incontinence to put a pad (cloth) on the place where the impurity exits, but it is not obligatory, and that it is desirable for him to change the pad.
Al-Hattab, from the Maliki School of jurisprudence, said in Mawahib al-Jaleel:

“It is desirable as per al-Mudawwanah [authored by Imam Malik] to avoid that with a pad. Sanad said: ‘It is not obligatory, because he prays with the pad, and it carries impurity, as it is the case when he prays with his clothes. Sanad said: ‘It is desirable to change the pad?’ Al-Ibyani said: ‘It is desirable for him to do so when he prays, and he washes it. But according to the view of Sahnoon, it is not desirable [for him to do so].’” [End of quote]

For more benefit on someone suffering from urine incontinence, please refer to Fatawa 387577, 365380, 343018, 349147, 354916, 358042 and 361150.

Allah knows best.