In Quick Reply you can type the emoticon's code, which is its name between two colons :name:, if you know it. If not you'll have to click on Reply or edit your Quick Reply as noted above.
(You can learn the names by rolling your mouse over the emoticons in other posts or in the menu when you are in Reply mode or editing a Quick Reply.)
The name of the emoticon in totoro's post is thumbsup which I got by hovering my mouse over the emoticon. So now I type colon-thumbsup-colon :thumb: and there he is.