I'm starter of Wordpress. And I really want to know how to add featured image of post using HTML.

I know there is default function to add featured image on sidebar. But on that, I can only add images that already exist on my media. But I want to add by HTML code. That code is

<a href="https://~~~" ></a><img src="https://~~~" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" />

I want to add that code to featured image. Please help me. I struggle for 2days to solve this...

Thank you.