My Numark Dj player has Cinch cable [...] and is conected to the laptop by jack.
Please do not make that connection, it can harm your laptop. The RCA or Cinch outputs are meant for connection to an amplifier, audio mixer, recording device etc. The voltage of that signal is way higher than microphone level that your laptop expects.
Actually, I was asking about the laptop's possibilities. According to
this Numark User Guide the USB port is meant for connection to a computer (
source).
After connecting via USB: have it installed correctly under Windows first, so make sure you can hear it via the internal speakers, or headphones. (Sometimes after plugging in a usb device for the first time, it will take a few moments...)
Within OBS Studio: add an Audio Input Capture source to your scene(s). At the properties of the source, at device: select your Platinum FX.