Make Featured Image link to External URL

I want some featured images of the posts on my home page to link to an external URL.

I’ve been searching pages and pages of Google results to try and find a way to do this. The only useful posts I found were too difficult for me because they included changing code in .php files. What I got from this is that I need to add some code either in the content.php or single.php file, which will check if the given post has a custom field. I then create a custom field with the link set as the value for each post where I want the featured image to link to an external URL.

Can anyone explain in a newbie-friendly way how I can do this?

Thank you!