Webform default value of select list
If you fx want to set the default value using, given it is passed as a parameter in the URL:

function hook_form_FORM_ID_alter(&amp;$form, &amp;$form_state) {
  $params = drupal_get_query_parameters();
  if (isset($params['my_param'])) {
    $form['FIELD_NAME']['und'][0]['#default_value'] = $params['my_param'];
  }
}


https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_form_FORM_ID_alter/7