Turn on HDR permanently in Windows settings. Or manually toggle it on before opening the video by pressing WinKey+Alt+B. Turn off HDR passthrough in MPC Video Renderer settings. Then HDR video will get tonemapped into SDR video.
Information on configuring HDR and Upscaling on your HTPC, for multiple platforms and media players (MPC VR, MPC-HC, MPC-BE, madVR, Kodi, Plex, VLC, Potplayer)