Configure projects page), and select "Yes" for "Enable prefix". Then - in the box below - specify a prefix that will be used (e.g. "CRAP").
Now all issues for this project uses the prefix specified. Issue #1 will be called "CRAP-1", issue #2 "CRAP-2", etc. This is also backwards-compatible, so you can change it any time you want, and it will be valid for all your issues in that project.