If it's someone famous, I find it quicker to just google it, e.g., googling "wikitree robert the bruce" gave me his profile as the first result:
Bruce-129
At the top of your profile, at the top, there is a Spy glass, do an enter, the next screen do a enter on that spy glass https://www.wikitree.com/wiki/Special:SearchPerson Put in the last name, the page will load with all the people with that last name. Go the top you will see Quick links for members: .click the alpha order and start seaching.
Good Luck on your search!
If it it someone notable like Robert the Bruce you could use the Wikidata link and see if someone has linked a WikiTree profile to it. The Wikidata page for Robert the Bruce is https://www.wikidata.org/wiki/Q187312 which has a WikiTree profile attached (At the bottom) of Bruce-129 .