OBD 11 standard for 12V is
Pin 16 battery +
Pin 4 chasis ground (Pin 5 signal ground).
There is no OBD 11 standard for ignition live, although some manufacturers support this. However, there are OBD 11 microcontrollers available, that operate at 5V 3A / 12V 3A and have ACC and Surveillance modes, such as that seen in the example link below. It is important to research and buy a reputale device, rather than the cheapest available!
https://www.amazon.co.uk/Adapter-Hardwire-Charger-Surveillance-Protection/dp/B0DR19H2QQ/ref=asc_df_B0DR19H2QQ?tag=bingshoppinga-21&linkCode=df0&hvadid=80745591663822&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=69182&hvtargid=pla-4584345058719349&psc=1&msclkid=d14efc9cbb2c1da7757b95d702196ce9
If going down the route of a basic 12v OBD11 power adapter, its imprtant the output lead is protected by a cartridge fuse, as OBD pin 16 battery + is often not fused and even if it was, it would be rated at far higher Amps than that of your dashcam.