Using a NZXT S340 elite and asus strix b350f motherboard, one of the front panel connecters is a hdmi cable but I dont know where to put it on the motherboard
That is for a VR (Virtual Reality) headset. You can connect it to your graphics card HDMI out port to use as an extension of that port. If you don't have a VR headset then you don't need to connect it to anything.