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 hover over the More button to expand it. Click the HTML button in the bottom right hand corner and a HTML block should appear (the circled block on the left with "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.