select tag inside form

Hello.
It’s possible at the any paid versions of the Booking Calendar.
You can configure the form fields in these versions on the Booking > Settings > Fields page.
Please read more about the booking form fields configuration here
Also you can test the live demo of those versions at this page.
Kind regards.

Hello.
You can try to do that in this file ../booking/lib/wpdev-booking-class.php
in the content of this function:
function get_booking_form($my_boook_type) {

by adding your code after this code (in the same way):

$my_form.='  <div class="control-group">
                      <label for="details" class="control-label">'.$booking_form_field_label5.(($booking_form_field_required5=='On')?'*':'').':</label>
                      <div class="controls">
                        <textarea rows="3" name="details'.$my_boook_type.'" id="details'.$my_boook_type.'" class="input-xlarge'.(($booking_form_field_required5=='On')?' wpdev-validates-as-required ':'').'"></textarea>
                      </div>
                    </div>';

Also you will be need to add the code for showing the data from that field in the content of this function get_booking_form_show() , which located in the ../booking/lib/wpdev-booking-function.php file.
after this code:

$booking_form_show.='<strong>'.$booking_form_field_label5.'</strong>: <br /><span class="fieldvalue">[details]</span>';

Please note, if you will modify the source code of the Booking Calendar, we will not guaranteed the correct work of plugin and do not support it.

I was mentioning that in my previous reply at the bottom of the reply.
It’s inside of the wpdev-booking-functions.php.
Again, please note, we do not support such modifications.