As a rule, you cannot determine which Stack Exchange site is appropriate for asking a question by simply noting that it is "about" or "focused on" a specific tool, product, service etc.
What matters is what kind of question you are trying to ask about that thing.
In your case, the question was off topic at Stack Overflow because you weren't asking about how to make your IDE use GitHub Copilot properly, or about how to get the output from Copilot into your program, etc. Your question is about prompting the AI that Copilot provides, in order to get the result you want.
This is also off topic at Super User because you weren't asking about, for example, how to get the AI to see your prompt (i.e. something about "how to use your computer" that goes beyond what ordinary computer users have to worry about), but instead about what to put in the prompt.
For questions like this, please consider GenAI.