Use behat extension to fill in a Drupal autocomplete field and select the right option which appears.

FeatureContext_fill_in_drupal_autocomplete.inc

  /**
   * @When I fill in the autocomplete :autocomplete with :text and click :popup
   */
  public function fillInDrupalAutocomplete($autocomplete, $text, $popup) {
    $el = $this->getSession()->getPage()->findField($autocomplete);
    $el->focus();

    // Set the autocomplete text then put a space at the end which triggers
    // the JS to go do the autocomplete stuff.
    $el->setValue($text);
    $el->keyUp(' ');

    // Sadly this grace of 1 second is needed here.
    sleep(1);
    $this->minkContext->iWaitForAjaxToFinish();

    // Drupal autocompletes have an id of autocomplete which is bad news
    // if there are two on the page.
    $autocomplete = $this->getSession()->getPage()->findById('autocomplete');

    if (empty($autocomplete)) {
      throw new ExpectationException(t('Could not find the autocomplete popup box'), $this->getSession());
    }

    $popup_element = $autocomplete->find('xpath', "//div[text() = '{$popup}']");

    if (empty($popup_element)) {
      throw new ExpectationException(t('Could not find autocomplete popup text @popup', array(
        '@popup' => $popup)), $this->getSession());
    }

    $popup_element->click();
  }

You can simply use Given I fill in "My Autocomplete Value" for "my_autocomplete_field"