You use entity_autocomplete as #type for the form field.
CommentForm::form() contains such a form field, for the administrator users to be able to change the author of a comment. This is the code it uses.
// The uid field is only displayed when a user with the permission
// 'administer comments' is editing an existing comment from an
// authenticated user.
$owner = $comment->getOwner();
$form['author']['uid'] = [
'#type' => 'entity_autocomplete',
'#target_type' => 'user',
'#default_value' => $owner->isAnonymous() ? NULL : $owner,
// A comment can be made anonymous by leaving this field empty therefore
// there is no need to list them in the autocomplete.
'#selection_settings' => ['include_anonymous' => FALSE],
'#title' => $this->t('Authored by'),
'#description' => $this->t('Leave blank for %anonymous.', ['%anonymous' => $config->get('anonymous')]),
'#access' => $is_admin,
As you can see from the code, you can also choose to include the anonymous user.
The result is shown in the following screenshot.