Add preview button on Taxonomy add/edit term form
Preview button added by node module in NodeForm as action in actions method:

$element['preview'] = [
      '#type' =&gt; 'submit',
      '#access' =&gt; $preview_mode != DRUPAL_DISABLED &amp;&amp; ($node-&gt;access('create') || $node-&gt;access('update')),
      '#value' =&gt; t('Preview'),
      '#weight' =&gt; 20,
      '#submit' =&gt; ['::submitForm', '::preview'],
    ]; 


The Taxonomy entity form created by TermForm.php so i think you should:


Extends TermForm.php in another Class.
Add actions method.
Create a preview method like the node preview method but for the term.
Add a route subscriber to tell drupal to load your form not the default.