You can find the List ID (Group ID) in your Mailerlite account from here:
MAILERLITE - Profile > Integrations->Developer API->Subscriber Groups
And you should make sure that each field of the form has the same name in your Mailerlite account, to change field's name simply hover on the field inside the builder and click on the settings button on it, the from the popup you can change field's name, placeholder and set if it's required or not.