yes you need resistor.
R = resistor, 10k ohm.
read more: https://www.electroschematics.com/12012/rotary-encoder-arduino/
about volume up/down, I’m not sure how BDN9 firmware read the encoder value, do they use QMK? with QMK, you need a slight delay between registering keyup / keydown. (source: https://www.reddit.com/r/olkb/comments/9jzbg1/help_with_rotary_encoder_code/e8elobo)