How can I add a textbox in Site Information configuration
You need to add your own module if you're looking to alter this form. You'll need 2 things:


An alter form hook function to add the text field.
A submission handler to process the data.


Something like this...

function mymodule_form_system_site_information_settings_alter(&amp;$form, $form_state, $form_id) {
  $form['site_information']['mymodule_site_organisation'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('Site Organization'),
    '#default_value' =&gt; variable_get('mymodule_site_organisation', ''),
  );
  $form['#submit'][] = 'mymodule_handler';
}
function mymodule_handler($form, &amp;$form_state) {
  variable_set('mymodule_site_organisation', $form_state['values']['mymodule_site_organisation']);
}


Once implemented, you can reference your custom value by using variable_get('mymodule_site_organisation', '') anywhere you need it.