Escribir tabulador (Python) en textarea HTML

/* Javascript */

document.getElementById('textbox').addEventListener('keydown', function(e) {
if (e.key == 'Tab') {
e.preventDefault();
var start = this.selectionStart;
var end = this.selectionEnd;
// set textarea value to: text before caret + tab + text after caret
this.value = this.value.substring(0, start) +
"\t" + this.value.substring(end);
// put caret at right position again
this.selectionStart =
this.selectionEnd = start + 1;
}
});

<!--HTML-->
<input type="text" name="test1" />
<textarea id="textbox" name="test2"></textarea>
<input type="text" name="test3" />