I tried again, and the widget worked!
I saw only a simple text link because it was wrong. The correct address starts with https://.
(The example code start with http:// but Mailtrain on Cloudron uses https://)
After getting this worked, I'm started to want to customize the widget.
(e.x. I want to use "Thank you for subscribing!" instead of the default message says "It looks like you are already subscribed to this list.")
There's a file called widget-subscribe.hbs where we can customize the appearance of the widget.
However, it is not accessible on Cloudron.
Is(Will be) there any way to customize the widget?