Edit in JSFiddle

$.fn.EnableInsertAtCaret = function() {
$(this).on("focus", function() {        $(".insertatcaretactive").removeClass("insertatcaretactive");
     $(this).addClass("insertatcaretactive");
});
};


function InsertAtCaret(myValue) {

    return $(".insertatcaretactive").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(endPos, this.value.length);
            this.focus();
            this.selectionStart = startPos + myValue.length;
            this.selectionEnd = startPos + myValue.length;
            this.scrollTop = scrollTop;
        } else {
            this.value += myValue;
            this.focus();
        }
    })
}
$(".inputHolder,#txtInput").EnableInsertAtCaret();


$('#test').click(function() {
    InsertAtCaret('test');
});

<textarea class="inputHolder" cols="30" rows="10"></textarea>
<textarea class="inputHolder" cols="30" rows="10"></textarea>
<input type="text" id="txtInput"  />
<br/>
<br/>
<button id="test">Insert "Test" at Cursor position</button>

.insertatcaretactive
{
border:1px soild #1e90ff;
}