Add the Violent Monkey extension to your browser.
Create a new user script,
copy and paste all the code from AddYourCertificateToLinkedIn.js on the new user script, save it, open https://www.boot.dev/u/<your_user_here>,
click on any completed course, the button should now show if the script is active.
Open https://www.boot.dev/u/<your_user_here>, click on any completed course,
go to GreasyFork
(alternatively you can search for it by clicking on the extension and Find Scripts for this site,
currently there is only mine, it's called Button to add boot.dev certificate to linkedin) then you click on the
green Install button, install it, refresh the boot.dev page.
By following any of the 2 options you should have the script added and running. If you have any questions ask me here.