It's in the topmost panel of the question authoring form, as shown in the figure below. However, it's a new feature and may not be in your version of CodeRunner. It's only relevant when you're randomising questions, so if this is still about adding prolog to CodeRunner you can ignore it.
Forum
Question Authors' Forum
Adding Prolog in Coderunner
Acest forum limitează numărul de intervenţii pe care le puteţi realiza într-un interval de timp - în prezent această limitare este de 5 intervenţii în interval de 1 zi