Thanks for the reply, @kellymetal!
I assumed this would be the answer–just wasn’t sure if there was a way to initialize the API or load the WordPress plugin architecture on a website that has WordPress, but just isn’t being used on the page in question.
I can’t use an iframe form in this case since the contact page’s primary function is to send an email (I was hoping to create something like the MailPoet checkbox that can be added to a WordPress comment). But this would be a good idea if I just wanted another signup form somewhere.
Maybe I can re-create the page in WordPress and use htaccess to make the page look like it’s outside of the blog. For now, I’ve added the checkbox and, if checked, a note is added to the message to let me know I should add the user manually to my MailPoet newsletter list.
Thanks again!