Chickens likely domesticated themselves the same way wolves did which is basically by hanging around our settlements and eating our refuse.
Living with chickens in this sort of way was likely happening way before the estimated times of 8,000- 10,000 years ago. The relationship was likely already long established it was just during that time some chickens started developing smaller/weaker adrenal glands which caused them to become much easier to raise and handle which eventually lead to the domesticated chicken we know of today (and how that happened so suddenly is a completely different but very interesting topic)
Also due to this you could argue that there isn't any true wild red jungle fowl left untouched and uninfluenced by humans anymore. Not only due to constant cross breeding with domestic chickens but because red jungle fowl are still doing what their ancestors did in their current range, if there is a town or village nearby the bravest junglefowl will still choose to intermingle with the village and eat the refuse, agricultural byproducts, and waste. People will still catch and care for these "wild" birds like their ancestors did. This isn't to say we shouldn't try our best to preserve the wild red jungle fowls wild genetics, their should be populations left to be in their natural environment but it's likely they are not truly same wild birds they once were thousands of years ago and honestly that's OK because thats how its been for thousands of years.