following other guides, I added in the page form to

/httpdocs/vendor/magento/module-contact/view/frontend/templates/form.phtml

and I added the code

<div class="field selezione required">
        <label class="label" for="subject"><span><?php /* @escapeNotVerified */ echo __('Come hai conosciuto AvaloItalia?') ?></span></label>
        <div class="control">
            <select selectname="selezione" id="selezione" title="<?php /* @escapeNotVerified */ echo __('Come hai conosciuto AvaloItalia?') ?>" data-validate="{required:true}" value="ciao">
              <option value="facebook">Facebook</option>
              <option value="google">Google</option>
              <option value="altro">Altro</option>
            </select>               
        </div>
    </div>

I adjusted the template that I get email but the field is always empty.

What am I doing wrong? how can i fix it?