To send the submitted date to your Mailerlite account you should have the following information:
Your API key from here - https://app.mailerlite.com/integrations/api/
List unique id from here - https://app.mailerlite.com/integrations/api/
First you should open the settings of the project and then open the "Forms" tab, then choose Mailerlite and input the API key in the corresponding field at the end of the page.
After that you should open your project inside the builder and click on the "Form Settings" button in the settings of the column which contains the form, then from the right sidebar open the "Settings" tab and choose "Mailerlite" as form type and input the list id in the "List ID" field.
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.