Here is a simple example…
function send_integromat_webhook() {
var subject = 'Test';
var recipient = 'user@example.com';
var body = 'Just testing';
var obj = {
subject: subject,
recipient: recipient,
body: body
};
var json = JSON.stringify(obj);
var url = 'https://hook.integromat.com/<HOOK_ID>';
var options = {
method: 'POST',
payload: json,
contentType: "application/json; charset=utf-8",
};
var response = UrlFetchApp.fetch(url, options);
}