Inserting newsletter signup form: page-inject?

My goal is to be able to easily place a newsletter signup form in various pages. I have a whose header contains a working form that adds new subscriptions to Mailchimp. As a standalone page, this form displays and functions correctly.

I thought I could use the page-inject plugin, but the page-inject shortcode renders the raw HTML of the form on the page where I use it, instead of displaying the form.

I understand that I could create a custom plugin with shortcode to inject the form wherever I need it, but that is a bit beyond me. I’d be grateful for any ideas for making page-inject work for this purpose, or something else.

I’m using grav 1.7.rc8 by the way.