Source : http://yo3hjv.blogspot.com/2017/04/if-tap-for-external-sdr-on-yaesu-ft-840.html
Follow the red wire from the board in the back.
Source : https://forums.qrz.com/index.php?threads/yaesu-ft-840-first-if-tap-for-a-panadapter.553395/
Source : https://imgur.com/gallery/wV4Gf