Log-in to your Marketo account.
Select your landing page and click Edit Draft.
Drag in the HTML element.
Example in screenshot below
;and a single space. An example of what this would look like is:
It is likely that you have multiple Landing Page templates in use, be sure to add the code to all templates that have forms on them.
Sometimes when you edit the template for landing pages, you need to re-approve the pages the landing page is used by. This article explains how to mass approve.