To tack onto this, I’ve been in communication with the Mailerlite folks and I at least was able to get the form to submit and the test email is added to my list, but it still redirects to the MailerLite confirmation page. I’ll keep you posted if I find anything else out!
Here’s the overview video I sent to them, by the way:
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In case it helps someone else get a viable answer, here’s what the ML folks said:
Seems like, there should be some area where you should add “hidden” value, so that subscriber stays on your page and isn’t redirected to our embed form.
Example from integration with genesis e-news plugin
I guess, it should be something similar in your case.