Press Shift and Enter for a new line

<textarea id="message"></textarea>

const ele = document.getElementById('message');

ele.addEventListener('keydown', function (e) {
    // Get the code of pressed key
    const keyCode = e.which || e.keyCode;

    // 13 represents the Enter key
    if (keyCode === 13 && !e.shiftKey) {
        // Don't generate a new line
        e.preventDefault();

        // Do something else such as send the message to back-end
        // ...
    }
});