Some people are just less creative than others + can't write under timed pressure so this part of the exam would be tricky for a lot of people. I would just say, in the actual exam, spend 5-10 minutes just letting your mind run free for a bit, even close your eyes if you have to (but don't fall asleep in your exam) and just brainstorm ideas for a bit. Some people like to write down a plan for the story. If you really struggle to think of something, try reading a few short stories, they really help.