Submitting form data with fetch in JavaScript

fetch_form_submit.js

function postTagData(myUrl, documentId, fullText, tag, start, end) {
    let form = new FormData();
    form.set('document_id', documentId);
    form.set('full_text', fullText);
    form.set('region_start', start);
    form.set('region_end', end);
    return window.fetch(myUrl, {
        method: 'POST',
        body: form
    });
}