Hugh Lashbrooke

Sick of adding a image to your post content then also setting it as the featured image in a separate action? Of course you are!
This plugin solves this problem by adding a new button alongside the “Insert into post” button in the media manager modal. When clicked, this button will insert your selected media and then, if you have selected an image, set that image as the post’s featured image.
It is possible to select multiple media items to insert, so this plugin will set the last image in the selected set as the featured image. The button will appear in the media manager modal for all post types.
This plugin works with WordPress 5.0+, but it will only work with Classic block or in the Classic Editor. It does not currently work with the Image block (or any other similar ones).
Want to contribute? <a href="https://github.com/hlashbrooke/Instant-Featured-Image" rel="nofollow ugc">Fork the GitHub repository</a>.
Got some great ideas? <a href="https://wordpress.org/support/plugin/instant-featured-image" rel="ugc">Post on the support forum</a>.

Instant Featured Image

Set an image as the featured image at the same time as you insert it into the post content.

Mohamed Tawfik

This plugin let you add a featured image to a post using external URL.
You can use images from Flickr, google images… etc just in one click. 
No need to download the image and re-upload it again.
The image will be processed and thumbnails will be generated just like if you uploaded the image your self from your PC.
It is working with native and custom post types.
It is available now in two languages:
1- English 
2- Arabic
You can translate it to any languages.

WK URL 2 WP Thumb

This plugin let you add a featured image to a post using external URL.

Instant Featured Image vs WK URL 2 WP Thumb