How to get or set boundary in multipart/form-data from FormData?

As in topic: I would like to set up my own boundary parameter for Content-Type when I work with FormData class in JavaScript.

Expected header:

Content-Type: multipart/form-data; boundary=------some-random-characters

My source code looks in the folowing way:

const requestData = new FormData();
requestData.append('file', file); // file from File API

const response = await fetch('/backend/upload', {
    method: 'POST',
    headers: {
        'Content-Type': 'multipart/form-data; boundary=------some-random-characters',
        'Accept': 'application/json'
    },
    body: requestData
});
var responseData = await response.json();

When I execute the source code, header Content-Type is correct, but in body I have:

------WebKitFormBoundary2lZSUsxEA3X5jpYD instead of ------some-random-characters.

Request screenshot from Google Chrome DevTools:

Any idea how to get or set boundary for FormData?