fetch than one specifically oriented at how to use Netlify or its tools. That’s why @perry has to say
We can’t, as a general rule, help you learn, write, or debug code (there are sadly only 8 support engineers for hundreds of thousands of sites worldwide)
fetch takes a second argument of options for request data, which should contain a
body for the actual body of the request, which ought to be stringified JSON. An array is a valid json value provided it has a valid key.
Give this a thorough read: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API