To add custom HTML code, click New email/template / Basic templates / My HTML.
In the drop-down window, insert your code.
Please note that when you insert custom code, all the default editor options are disabled. They will not have the necessary classes for this, so you can only change the button images and copy (if there are any)
To adapt your code to the editor menu options, add our classes.
For more details on how to do this, read this post.