It is possible, but you have to find the config.ini for devmode to enable hdr. The location of the file depends on the folder where rF2 core is installed.
My core folder is "C:\rFactor2" and the config.ini for devmode is stored inside this folder (standard users have full write permission to this folder).
The default path (C:\program files (x86)\rFactor2) is write protected for standard users. In this case you will find the config.ini at "C:\Users\WindowsUserName\AppData\Local\VirtualStore\Program Files (x86)\rFactor2".
Thanks for reply Tosch