Is it permissible to enter the bathroom and urinate before performing ghusl to cleanse oneself from janabah?

Regarding the prohibition of urinating in the bathing area: The prohibition is established from the Prophet, peace be upon him, and it was explained that most of the waswas (obsessive whispering/doubt) originates from it. The prohibition applies if there is a fear that some urine splash might reach the person, as waswas increases significantly then.

Regarding urinating before ghusl (ritual bath): There is no harm for someone who intends to perform ghusl al-janabah (ritual bath for major impurity) to relieve himself by urinating or similar acts before bathing. Then, after relieving himself, he must first perform istinja' (cleaning oneself after relieving oneself), and then proceed with the ghusl. The area of istinja' must be washed with the intention of ghusl al-janabah so that it is not overlooked.