I don't understand what this means. I use LLMs daily for my work in programming things, and they regularly will assert things that are not accurate.
Having a natural language interface where you need to go out of your way to specify that you want an accurate answer rather than just a plausible one defeats the entire purpose of it being a natural language interface for normal people. In certain professional contexts, it can be useful, but I don't buy it at all that it makes sense to ask everyone in their everyday lives to go out of their way to specify that they actually want correct answers to their questions.
You don't. It goes in the system prompt.