Step 2

Locate "Custom uploader settings" and open

Step 4

Fill in all the info as shown in the picture.
Name: screenshot.wtf (your choice)
Request URL: https://screenshot.wtf/upload/
If you want to use another domain, replace the screenshot.wtf part at Request URL.
At the body, fill in your username and password.
File form name: sharex
Image Uploader fills in automatically.