I looked at the Modernizr's docs and pics, and it looks like the board is using GPIO0 (zero) as the input. I've circled that in blue on the attached pic.
I can't quite read the value of the resistor in the pic, but the docs skirt around the 10k resistor value and Fietspomp also mentions 10K. If you want to follow his direction, then that is the resistor to change. He doesn't mention whether he replaced or added a 3.3K resistor. It may not make a difference if he added (which makes total resistance ~2.5k) or replaced (which makes total resistance 3.3k). It's a small difference. You might be ok with hand soldering the resistor across that SMD resistor (the one circled in blue) as SMD soldering can be tricky.
What I would do first would be to check the voltages across that GPIO0 and ground before and after you press the doorbell. That way you'll know for sure if GPIO0 is the correct pin.
I'm wondering why you're thinking about doing this mod? Is something not working? Did it used to work? If it just stopped working, then what's more concerning is that in the pic you've got something that looks like rust/oxidation or blown part (circled in green). I would be testing that optocoupler chip before worrying about adding resistors. Test the voltages before and after that chip.
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