Mailerlite can only be integrated through the use of another service. Integromat has a pretty generous free plan. You would use a webhook from Webflow to integromat then to Mailerlite.
Hopefully you can implement this and solve your issue.
I’m wanting to do the same thing, but I can’t figure out Integromat (Make). It also only seems to integrate with Mailerlite Classic, not Mailerlite, but I’m not sure if that’s a problem.
Where can I find out how to use webhooks to set this up?
It has some screenshots and walks through a high level, quick setup in Make (Integromat).
Beyond that, and specifically more about Webhooks + Webflow + whatever other service you want to use, here are a few tutorials and screencasts walking through just that: