What you are looking for is the descendant sticker. Here's a link to the Help page: Template:Descendant (wikitree.com)
Adding this to your profile will give you a sticker that says something like "Descendant of name of famous person" with the option to add more information, like "swam the English Channel in 1810."
To add this sticker you type the double curly brackets {{ then the word Descendant followed by the pipe | then id = followed by [[ then the Wiki-ID of the ancestor another | and their name followed by ]] and then more curly brackets }} If you want to add descriptive text that goes after the Wiki-ID (and its' brackets) and before the second set of curly brackets.
Stickers go under the Biography heading on a profile.