There are two ways to create an option dynamically :

1) Using the Option constructor and add() method

2) Using the DOM methods

1) Using the Option constructor and add() method :

let newOption = new Option('Option Text','Option Value');
const select = document.querySelector('select'); 
select.add(newOption,undefined);

2) Using the DOM methods :

// create option using DOM
const newOption = document.createElement('option');
const optionText = document.createTextNode('Option Text');
// set option text
newOption.appendChild(optionText);
// and option value
newOption.setAttribute('value','Option Value');

const select = document.querySelector('select'); 
select.appendChild(newOption);