How to create temporary select field?
Probably a hack, but this is how I solved it:

$form['field_brand_selector'] = array(
    '#type' =&gt; 'inline_template',
    '#template' =&gt; '{{ somecontent | raw }}',
    '#context' =&gt; [ 'somecontent' =&gt; 
    '
        &lt;label for="edit-field-brand-selector"&gt;Brand&lt;/label&gt;
        &lt;select class="" id="edit-field-brand-selector" name="field_brand_selector" readonly="readonly"&gt;&lt;/select&gt;
    '],
    '#weight' =&gt; 8,
);


This way Drupal doesn't validate the field I added.