I'm not sure if there's a repo approved way of activating it, but the devs left in an old main menu track called "main_theme_old" which seems to be Fall Fountain by gabester. In Code: [game install path]/lua/ge/main.lua line 605 you can change the track name. Keep in mind though that this file is likely to be changed in the next update, which will overwrite it.
could you help decode the line? where I need to place file, in which format and how to write the soure location? Looking at the placement of this file, it seems to be hidden somewhere, but could find it...
I'd recommend a text editor line notepad++ so that you have line numbers. You only need to edit the file, so it needs to stay in the same format. If you go to BeamNG in your steam library there will be a cog icon ⚙️on the right where, if you follow the image below, you will go to the [game install path] in windows explorer. If your game breaks, or you mess anything up just verify the integrity of your game files using steam (cog icon > Properties > Installed files). It will return everything back to normal.
