How to enable IMCE in a field of a custom block
You can provide an alternative URL input(type=text) for IMCE integration.

You just need to include drupal.imce.input library and set the input class name as "imce-url-input". Also see imce.input.js file

if (\Drupal::moduleHandler()-&gt;moduleExists('imce') &amp;&amp; \Drupal\imce\Imce::access()) {
  $form['background'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; $this-&gt;t('Background URL'),
    '#attached' =&gt; array('library' =&gt; array('imce/drupal.imce.input')),
    '#attributes' =&gt; array('class' =&gt; array('imce-url-input')),
    '#default_value' =&gt; isset($this-&gt;configuration['background']) ? $this-&gt;configurati‌​on['background'] : '',
  );
}