All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

Discharging semen out of penis is a condition that makes Ghusl obligatory, but if semen didn`t come out or no trace of it was seen then Ghusl isn`t obligatory."Even if a person felt semen coming out but held it in by pressing the penis with hand and nothing came out, Ghusl isn`t obligatory."{Moghni al-Mohtajj, 1/215}.

If semen came out in any form, even with urine, then performing Ghusl is obligatory. "Once semen came out of penis or once the woman saw the white discharges coming out of her, whether that was before or after urinating , Ghusl is obligatory."

In conclusion, whenever you(Asker) are sure that semen was discharged with your urine then you must perform Ghusl. And Allah knows best.