Hi,
The text area is not a template. It's PHP code. So you need to use PHP code, like in the preload area.
If you want to make it easy, you can empty the text area completely. In that case, it will automatically use the HTML version output and strip the HTML to generate the text version. That way, you don't have to handle that yourself.
Otherwise, it's complex if you want to do that.
The custom fields data is not loaded there, so you need code to do that first.
Please understand that such custom coding is not part of user support. If you need a developer to help you with that customization, you can contact our partners:
www.hikashop.com/home/our-partners.html