1/ You question belongs to Products, please move it there. The specified item was not found. is for generic Marketing questions.
2/ You do not create a Landing page from some HTML code in Marketo*. This is not possible. You have to create a template first and then use it to create a landing page. You create a template in the design studio.
A template will need to have at least some editable zones so that it can reused to create multiple landing pages.
I have seen people creating a new template for each LP and hard code completely the template on it. This is not a good practice because you will end up with a very high number of templates, each of them used only once, which basically denies the "Template" concept.
*This can be done for emails but nor for landing pages.
Knak is great for building out landing page templates that are customizable in Marketo. Very reasonable pricing as well.
Like Gregoire has mentioned, its not possible to create a landing page from scratch in Marketo. It requires HTML coding.
Marketo has some templates available as a starting point. You'll still need to go into the code to update it for your brand, but that's a good spot to start.
If you're looking for something more robust, you can checkout a company I started called Knak. We have templates that are build specifically for Marketo as well, the difference being you don't ever need to look at code in order to be able to customize it for your brand. We do have a full-blown email builder where you can build an email from scratch, but we don't have this for landing pages... yet.
In the meantime if you want a true start-from-scratch and build a page, checkout Unbounce. They have a Marketo integration as well. Your pages won't live in Marketo (which has several downsides), but you can at least pass any leads that fill out your form through the API.
Hope that helps!
Unbounce's API integration is broken. Don't use it, use the forms endpoint.
P. S. Forum rules have changed and we're not allowed to refer to our own products/services anymore.