thickbox.cmb2.php

<?php
add_action( 'cmb2_admin_init', function() {
  $prefix = 'mk_add_thickbox_';
  $metabox = new_cmb2_box( array(
    'id'            => $prefix.'box',
    'title'         => 'Signature',
    'object_types'  => array( 'post', ),
  ) );

  $metabox->add_field( array(
    'name' => 'Signature',
    'desc' => 'Draw a signature',
    'id'   => $prefix.'field',
    'type' => 'mk_add_thickbox',
  ) );
});

add_action( 'cmb2_render_mk_add_thickbox', function() {
  add_thickbox();
  ?>
  <div id="mk_thickbox" style="display:none;">
    <h1>Signature</h1>
    <p>Draw your signature below..</p>
    <div style="background: white; height: 200px; width: 100%; border: 1px solid #333; display: flex; justify-content: center; align-items: center; font-style: italic;">Draw your signature..</div>
  </div>
  <a href="#TB_inline?width=600&height=550&inlineId=mk_thickbox" class="thickbox">Enter signature..</a>
  <?php
}, 10, 5 );

Custom field for CMB2 which opens thickbox.