<script type="text/javascript">
function wstaw(myField, myValueStart, myValueEnd)
{
    //IE support
    if (document.selection)
    {
        myField.focus();
        sel = document.selection.createRange();
        sel.text = myValueStart+sel.text+myValueEnd;
    }
    //MOZILLA/NETSCAPE support
    else if (myField.selectionStart || myField.selectionStart == 0)
    {
        var startPos = myField.selectionStart;
        var endPos = myField.selectionEnd;
        myField.value = myField.value.substring(0, startPos)+ myValueStart+myField.value.substring(startPos,endPos)+myValueEnd+myField.value.substring(endPos, myField.value.length);
    }
    else
    {
        myField.value += myValue;
    }
}
</script>

wstaw('nazwa_formularza.nazwa_textarea', '<center>',' </center>');