Sorry Ruben, my first reply was typed from a mobile device, and I neglected to see which forum this was in.
further from my copy/paste buffer
1) click user list, located at the top of every page
2) type display name to search for
3) select the one you want from the list
4) click on "private message"
in addition, there is Aax code on UBBDev regarding this for UBBT 7.2.1 (2007), but it is an absolute mess. The archive includes multiple javascript files with unrelated code, and an exe file.
https://www.ubbdev.com/forums/ubbthreads.php/topics/315034/7-2-1-ajax-search-member.html
