That method is only possible in the Gutenberg Editor.
If you wanted to do that in the Classic Editor, you would need to switch to the code editor view, where you can see the HTML and then add your own HTML like so:
<div class="scroll-box">
<div class="scroll-box-inner">
Your content goes here
</div>
</div>
I have made some little tweaks in it (width and bg colour for example). But how do I make this section align: right? and once done, will I able to write something on the left side (blank part)? if yes, then how?