You can do that with the "Wildcard search" on the main page, but you need to take two steps:
- I searched for "j* challenge Czechia" for all challenges containing the letter "j"
- Then in the resulting table, I filtered the results (the magnifying glass on the bottom left) with "Cache name" "begins with" "j"
That's it.
You can obviously omit the location or choose the one you are interested in, but in the world, there is probably more challenges that you can show in this list.