PHP “Do action”

Can you tell me a bit more about what you want to do, as we have a number of different action hooks and filters available for specific purposes, we also have JS hooks for the form submission if that makes more sense for your case.

We have a lot of code snippets here if you want to browse some by yourself:
https://github.com/ibericode/mailchimp-for-wordpress/tree/master/sample-code-snippets/forms

Hope that helps. If you have any questions, please let me know!