I have created a custom block, but there is no option to add content or any body part.
How do I add content to a block?
I think you first want to define the fields of a block content type. You'll find this at this url:
In the admin UI this is not so easy to find like the node content type at the top level of the admin structure. The block content types are two or three levels deeper in the block layout. To access above link make sure you have the Custom Block module enabled.
I am able to get it working with the response provided by @Aram Boyajyan
To add a custom block, you need to enable Custom Block module which allows the creation of custom blocks through the user interface. Then in Structure, Block layout, when placing a block, you should have access to the new option Add custom block which should allow you to create blocks with HTML content (at the first time, you'd have to create a new block type for it).