var xhr = new XMLHttpRequest();
var url = "http://localhost:12345/endpoint";
xhr.open("POST", url, true);
xhr.setRequestHeader('Content-Type', 'multipart/form-data');
xhr.send('{"hello":"world"}');
No trickery required, it just does it.
It seems to agree with you. There's nothing I could find named imposta.exe or aryala.exe, but I did find installa.exe.
[0] https://discmaster.textfiles.com/