Q:

Display a block based on current select option

I have a select field that holds two options. When the second option is chosen, I want to display a block.
I know that with form API I should be able to do this. I'm just not sure on where to start. Any help getting me going would be greatly appreciated.
So far I have the code below.
function MODULE_form_user_register_form_alter(&$form, &$form_state, $form_id) {
    $form['account']['type'] = [
      '#title' => 'I am ordering for',
      '#type' => 'select',
      '#options' => [ 'Myself' => 'Myself', 'Someone' => 'Someone else' ],
      '#weight' => -100
      ];
}

A:

Don't try to add the fields in a form alter hook. Do this in the field UI of the user entity /admin/config/people/accounts/fields. To change the visibility of fields dependending on the content of another field use
Conditional Fields
or the Form #states API directly, see How can I hide or show fields using states with hook form alter.
EDIT: The form #states api did the trick.

