I tried to do it with the keyboard but it does not work (Low C + E) In any way I would not turn on and hold the appropriate keys, I still see only the OBX8 logo on the screen
Yes, there must be another combination of buttons to hold whilst powering up the unit to get that Bootloader Mode on the desktop version. On the Prophet REV2, you hold 3 buttons (Unison, Hold, Glide) and power up the synth to get in Bootloader Mode.
Perhaps you could try to press Hold, Chord, Unison, and power on the OB-X8 desktop? If it works the display should show --Bootloader-- Send Sysex File
Remember that in this mode, only MIDI connection will work for sending the OS to the device.