When typing in the Skype Client for Linux / Ubuntu, the spell checker makes a lot of mistakes. Being based on Electron, it should allow adding words to the dictionary in the same way that Chromium does. However upon right-clicking a words, no context menu to add the word appears.
The Skype for Linux client delivered by snap keeps its custom dictionary at /home/sid/snap/skype/common/.config/skypeforlinux/Custom Dictionary.txt however this file does not exist by default.
If you have google-chrome installed, this will pre-populate it with your existing custom dictionary words:
$ ln -s ~/.config/google-chrome/Default/Custom\ Dictionary.txt ~/snap/skype/common/.config/skypeforlinux/
If instead you are using chromium-browser, use this instead:
$ ln -s ~/.config/chromium/Default/Custom\ Dictionary.txt ~/snap/skype/common/.config/skypeforlinux/
After this, each time you add a new word to the dictionary in your browser, it will appear after the next time you restart the Skype client.
It's better than nothing!