Normally the case comes with the cable that goes to the mainboard.
Since you are missing that cable you'll have to assemble your own.
First step would be to identify the pinout which can be easily done by taking out the front panel pcb and looking at the traces
or when in doubt use a meter in continuity/resistance mode to identify which signal belongs on which pin.

After that you can assemble a cable that suits the pinout and allows you to hook up the mainboard to the front panel pcb.

I was aware of the "brute force" method, i kinda hoped that someone by any chance has the same front panel thingy and i could save some time analyzing pins and traces. 😀

In the end i did excactly that. I found two nice cables with single connectors, which came in handy to connect everything. Works fine.