Private Message Send API

What if you use an email template as a message?

I am going to the email_templates endpoint, grabbing the template I want, then send that as a private message. However, none of the strings are replaced in the message to the user. The message contains %{site_name} and ${base_url} inside the message text.

Just wondering how/when in the flow does Discourse substitute these (I couldn’t find any evidence).