If you have leftover sperm in your penis then the urine will flush out the extra sperm that is leftover in your penis.
But if you haven't ejaculated any sperm then when you pee the urine will not push or flush out any sperm inside your body.
Your sperm is located and produced in a different area than your urine.
Your urine is in your bladder and your sperm is not in your bladder even though the sperm comes out the same penis that urine comes out of.
You have to excite your penis by either rubbing it by masturbating or by inserting it into a women's vagina to cause the sperm to be produced and enter your penis.
So there's no way that peeing will flush out your sperm unless you have some sperm already in your penis then the urine or pee will flush the excess sperm out.