Is picking the "your own essay" on common app a bad idea?

I have read in some places that it is a trap to choose the option which asks you to submit essay of your own choice. However, I want to first write my essay and then designate the prompt to it. However, that might mean not applying to any of the prompts. What do you guys think?

That’s fine. The prompt is somewhat irrelevant. You are really trying to find the story you want to tell and fitting it to a prompt. Sometimes the prompts help students stumble upon a good story to tell, when they start answering a couple of the prompts and, for one of the prompts, end up writing a story they feel excited to tell that captures their personality. Colleges do not care about which prompt you choose as much as about what the essay reveals about you as a person.