How to use Skype secret and hidden emoticons in the my chat?
The hidden emoticons need to be typed in the main chat because you can't pick them from a list like you normally do. However, since they're too many to be listed here, I recommend checking the FAQ page provided by Skype where all the emoticons are listed including their chat activation code.
Full list of emoticons: https://support.skype.com/en/faq/FA12330/what-is-the-full-list-of-emoticons
Scroll down until you see the Hidden emoticons (Shhh, don't tell anyone!) title.