Hypertext Markup Language (HTML), is language used to describe the semantic content of web pages. This can be added using the custom HTML block. This block allows you to insert your code so you can fine-tune your content.
In order to add a custom HTML block, click on the Block Inserter icon.
You can also type /html and hit enter in a new paragraph block to add one quickly.
Detailed instructions on adding blocks can be found here.
Editing the Block
Once you add the block, you can add your code and view it right from the block options. You also have all the standard block options.
Block Settings Panel
While most blocks have specific options in the editor sidebar in addition to the options found in the block toolbar, the HTML block does not have extra options.