Jquery context menu move caret

Question

I'm trying to make custom context menu to insert images links to textarea.

The problem is, when I right click to textarea, caret is not moving to the point where i clicked:

jQuery.fn.extend({
    insertAtCaret: function(myValue) {
        return this.each(function(i) {
            if (document.selection) {
                //For browsers like Internet Explorer
                this.focus();
                sel = document.selection.createRange();
                sel.text = myValue;
                this.focus();
            } else if (this.selectionStart || this.selectionStart == '0') {
                //For browsers like Firefox and Webkit based
                var startPos = this.selectionStart;
                var endPos = this.selectionEnd;
                var scrollTop = this.scrollTop;
                this.value = this.value.substring(0, startPos) + myValue + this.value.substring(startPos, endPos) + this.value.substring(endPos, this.value.length);
                this.focus();
                this.selectionStart = startPos + myValue.length;
                this.selectionEnd = ((startPos + myValue.length) + this.value.substring(startPos, endPos).length);
                this.scrollTop = scrollTop;
            } else {
                this.value += myValue;
                this.focus();
            }
        })
    }
});

    $('textarea').entwine({

    oncontextmenu: function(e) {
        e.preventDefault();
        this.insertAtCaret('asdf');
        return false;
    },
});