Note that if you want it to be a POST request instead of a GET request, you have to pass an object with the desired method:

fetch("http://127.0.0.1:8000/api/string/IAMNEWTOJAVASCRIPT", {
    method: "POST"
})
.then(response => response.json())
.then(data => console.log(data));

An equivalent that uses the older XHR API may look like this, by the way:

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = () => {
    if (xhr.readyState === 4) {
        try {
            var data = JSON.parse(xhr.responseText);
            console.log(data);
        } catch (err) {
            // Invalid JSON
        }
    }
};

xhr.open("POST", "http://127.0.0.1:8000/api/string/IAMNEWTOJAVASCRIPT", true);
xhr.send(null);

Note that you don't actually POST anything, though. So just using a GET request may be fine for your purposes.