You didn't specify the version of BK you have, and mine isn't the most up to date by a long shot, but if yours is, all you have to do to find _anyone_ with particular criteria, including a particular given name, whether 1st, 2nd, or even 3rd, is do a word search for it.
On any individual person's screen, select the tab 'find' in the top bar, then pick 'Word Search' from the drop down menu. You'll have highly flexible search options explained. Just remember when you're done to close this subroutine.
If you are _sure_ the person's first name is what you want, you can bypass this routine by picking the simple F3 and enter the name followed by a question mark. And you'll get all by that first name, regardless of surname, but you won't get all who have that name as a 2nd given name except via the 'Word Search' option I mentioned above.