Try this code:

    var systemDescriptionRTETextArea = $("textarea[Title='System Description']");
    $(systemDescriptionRTETextArea).html("paste your text here");


**OR**

    
        $(document).ready(function() {    
        
        $('nobr:contains("Desc")').closest('tr').find('input[id$="TextField_spSave"]').val('Hello World !!!');
        });


try both and give your feedback.