adding drop-down subject (html select) to contact form
There is a select form element that you can add to your buildForm or hook override, depending on how you are building your form:

$form['example_select'] = [
  '#type' =&gt; 'select',
  '#title' =&gt; $this-&gt;t('Select element'),
  '#options' =&gt; [
    '1' =&gt; $this-&gt;t('Contact'),
    '2' =&gt; $this-&gt;t('Other'),
    '3' =&gt; $this-&gt;t('Customer Support'),
  ],
];


This is from the Drupal documentation: https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!Element!Select.php/class/Select/8.2.x.