Manual emails built using our drag&drop composer are made up of 'blocks' - for example, you can have a 'text' block, an 'image' block, a 'product' block, etc.
While we have multiple preset 'blocks' to make composing e-commerce emails easy, that doesn't mean that you can't choose to add your own flavour - besides changing your template (available from the Design page), we can also add your own HTML as a block.
Step 1: Add the block
Just scroll down to the bottom of the composer to find the "Add New section" options. Click the HTML button in the bottom middle and a HTML block will appear "Your custom html will appear here"
Step 2: Add your HTML code
This part is entirely up to you - however be sure to use HTML email best practices such as inline styling and mobile-first design.