Hi Martin,
Firsyly it seems that we need to wait a bit time for Timeline tab being loaded,
and the Note textarea element will only be generated after we clicked Enter a note input field, (there is also a delay before the Note textarea element being generated)
Thus I create two timer to wait for Timeline tab being loaded on form and Note field being generated, then I check if the Enter a note element is clicked,
after it has been clicked, then we wait a seconds for Note element being generated and grab it.
Obviously, the method works in unspported way.
function test() {
var elementIsClicked = false; // declare the variable that tracks the state
function clickHandler() { // declare a function that updates the state
elementIsClicked = true;
if (elementIsClicked) {
setTimeout(function() {
parent.document.getElementById('create_note_notesText').style.height = '200px';
}, 1500);
}
}
setTimeout(function() {
// Create a timer for Enter a note element being loaded
var element = parent.document.getElementById('create_note_placeholder'); // grab the Enter a note element
element.addEventListener('click', clickHandler);
}, 2000);
}

Regards,
Clofly