how to obtain the response headers of a Preflight request?

I am very new to web programming and I am trying to get the headers of the Preflight request. I have a server with node in javascript, from the server I serve an html. I need to retrieve the response headers in the html.
In the html to obtain the response headers of any request I use the following code:

fetch(signedRequest.url, {
    method: signedRequest.method,
    headers: signedRequest.headers,
    body: body
})
.then(response => {
    console.log('Response headers: ', response.headers);
});

However, I don’t know how I can get the headers response from the Preflight request since I don’t know where it is generated, can someone help me please?

I’m sorry if it’s an obvious question but I’m starting on web.