First, grab your ProveSource snippet code: https://console.provesrc.com/#/install
Landing Page
In your MailerLite dashboard, go to Sites -> Landing page -> click "Edit" next to the relevant landing page.
In the editor, under Special, drag the Code option to your page:
In the placeholder, delete the <div> tags and paste your ProveSource code, and save the changes:
Website
In your MailerLite dashboard, go to Sites -> Websites -> click "Edit" next to the relevant site:
Option #1:
The first option to add the code, is scrolling down in the settings, going to Analytics and custom code and pasting your ProveSource code under Header code injection, make sure you save the changes:
Option #2:
Another option is clicking "Save and continue" -> "Edit design":
On the right menu, to Blocks -> HTML code
Drag the code to the page and paste the ProveSource code, save the changes: