Hi! To enable HDR in most games you will need to set your game in fullscreen. Windowed or Borderless won't allow you to use HDR functionality.
You'll also need to use the DXVK_HDR=1 %command%
startup variable in steam to enable HDR in any Proton game. Proton 8 and 9 should support HDR without issuez although there are some games that might need additional configuration.
If you use an NVIDIA card, you might need to enable NVAPI with DXVK_ENABLE_NVAPI=1
Happy gaming! :D