Zur Info, so geht es:
var result = this.responseText;
var jsonResponse = JSON.parse(result);
var toke = jsonResponse["token"];
let formData = new FormData();
formData.append('_csrf_token', 'xxx.xxxx.xxxxxxxxx');
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][quantity]', 1);
formData.append('redirectTo', 'frontend.cart.offcanvas');
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][id]', 'e7b6a5682d2c4027bd93b62ddf8d5c25');
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][type]', 'product');
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][referencedId]', 'e7b6a5682d2c4027bd93b62ddf8d5c25');
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][stackable]', 1);
formData.append('lineItems[e7b6a5682d2c4027bd93b62ddf8d5c25][removable]', 1);
formData.append('product-name', 'Werkszeugnis 2.2/3.1');
formData.append('brand-name', '');
const xhr = new XMLHttpRequest();
xhr.withCredentials = true;
xhr.addEventListener('readystatechange', function () {
if (this.readyState === this.DONE) {
// console.log(this.responseText);
} else {
// console.log(this.responseText);
}
});
xhr.open('POST', 'https://domain.shop/en/checkout/line-item/add');
// xhr.setRequestHeader('Content-Type', 'multipart/form-data');
// xhr.setRequestHeader('Accept', '*/*');
xhr.setRequestHeader('sw-access-key', 'xxxxx');
xhr.setRequestHeader('sw-context-token', toke);
xhr.send(formData);