How to add custom code to pages?

What I understood is that using the Pages Module at /studio/builder_page.php I can add/remove/edit page blocks.

But, how can I add custom code to the page?

Here an example:

I added the field "Meta description" to the Post form. Now, how to add the following code the page?

<meta name="description" content="[VALUE OF THE META DESCRIPTION FIELD FOR THE SPECIFIC POST]">

Thank you