How can I add text into ckeditor textarea in drupal 7?
Assuming you want to set some value at the beginning -

Get the id of the form. https://www.drupal.org/project/get_form_id

Get the id of the field you want to alter.

Make a small custom module, with hook_form_alter() and set #default_value to that :

function YOURCUSTOMMODULENAME_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ( YOUR_FORM_ID == $form_id) {  
    $form['YOUR_FORM_ID']['YOUR_FIELD_ID']['#default_value'] = 'some default text'
  }
}


For adding some text after taht that, use jQuery.

$('#input-field-id').val('some text');