Set the message field as not required in a custom contact form
I've solved this issue this way:

function hook_entity_base_field_info_alter(&amp;$fields, EntityTypeInterface $entity_type) {
  if ($entity_type-&gt;id() == 'contact_message') {
    if (isset($fields['message']) &amp;&amp; $fields['message'] instanceof BaseFieldDefinition) {
      /* @var BaseFieldDefinition $field */
      $field = &amp;$fields['message'];
      $field-&gt;setRequired(FALSE);
    }
  }
}