The "3.3V" side of the button is likely an input pin with a pull-up resistor. To control it with an Arduino pin, use "pinMode(pin, OUTPUT); digitalWrite(pin, LOW);" to 'press' the button and "pinMode(pin, INPUT);" to 'release' the button. If you make the pin an OUTPUT and digitalWrite(pin, HIGH); your Arduino output will be shorted to Ground if anyone presses the button.
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