From the linked answer which does not use jQuery:
function insertRespuesta(id, attach)
{
var el = document.getElementById(attach)
if(el != null)
{
el.value += " #" + id + " ";
setCaretPosition(attach, el.value.length);
}
}
function setCaretPosition(elemId, caretPos) {
var elem = document.getElementById(elemId);
if(elem != null) {
elem.focus();
if(elem.createTextRange) {
var range = elem.createTextRange();
range.move('character', caretPos);
range.select();
}
else if(elem.selectionStart) {
elem.setSelectionRange(caretPos, caretPos);
}
}
}
Here is a working jsFiddle.