The code is the position of the marks in the barcode:
So the first digit is 3 because the mark is in the 3rd bar, then 6, then 9 because it's in the 18th position and we need a single digit, so 1+8, and the final one is 23, so 2+3. I was told people with colorblindness or certain screens can have a hard time noticing the marks in the barcode, I need to improve the contrast.
Anyway, the full code is 3695 :)
Thanks for playing!