nked below has screenshots for the procedure. See “If you want
  to open an app that hasn’t been notarized or is from an unidentified
  developer” section.</p>
  <ul>
  <li><a href="https://support.apple.com/en-us/HT202491">Safely open
  apps on your Mac - Apple Support</a></li>
  </ul>
  <p>If the plugin is still not working, try changing install location
  to <code>/Library/Audio/Plug-ins/VST3/</code> or
  <code>/Users/$USERNAME/Library/Audio/Plug-ins/VST3/</code> whichever
  still haven’t tried.<