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.

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 !!!');
    });

OR

$('nobr:contains("Rich Text")').closest('tr').find('input[id$="TextField_spSave"]').

val('<table border="1"><tr><td>Hello </td><td><b>World !!!</b></td></tr></table>');

try and give your feedback.