Switch automaticaly to 50hz refresh rate on PAL games

hello, is there any way to automatically set the refresh rate to 50Hz when starting pal games (c64, Amiga etc…) on retroarch? My monitor supports both 50Hz and 60Hz on HDMI:

$ xrandr 
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
HDMI-A-0 disconnected primary (normal left inverted right x axis y axis)
HDMI-A-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
   1920x1080     60.00*+  50.00    50.00    59.94  
   1680x1050     59.88  
   1600x900      60.00  
   1280x1024     60.02  
   1440x900      59.90  
   1280x800      59.91  
   1280x720      60.00    50.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
DisplayPort-0 disconnected (normal left inverted right x axis y axis)

It’s already implemented but only for Windows because that’s the only platform that has everything necessary for it already hooked up, apparently (according to the guy who hooked it up for Windows).

He also had this to say:

AFAIK for linux one must force video_refresh_rate to 50 on start, and it may or may not switch the display rate also

thx for reply. i have tried to set video_refresh_rate = “50.000000” on retroarch.conf but monitor video resolution remains 1920x1080x60Hz