JQUERY : ADD TEXT ANYWHERE IN INPUT WHEN CLICK

    jQuery("#btn").on('click', function() {
        var $txt = jQuery("#txt");
        var caretPos = $txt[0].selectionStart;
        var textAreaTxt = $txt.val();
        var txtToAdd = "stuff";
        $txt.val(textAreaTxt.substring(0, caretPos) + txtToAdd + textAreaTxt.substring(caretPos) );
    });

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>

<textarea id="txt" rows="15" cols="70">There is some text here.</textarea>
<input type="button" id="btn" value="OK" />