To add a HTML snippet, click on the Add Block button and select the Custom HTML block. Alternatively, you can start typing
/html in a new paragraph block, then press enter.
Every block comes with unique and block-specific controls that allow you to manipulate the block directly right in the editor. The custom HTML block is pretty straightforward so it only has the more options control.
Once you add the custom HTML block, you can add your code and view it right from the block options.
Clicking on the cog icon next to the publish button will toggle the visibility of this panel. While most blocks have specific options in the editor sidebar in addition to the options found in the block toolbar, the custom HTML block does not have extra options.