wow, I did not know that slackpkg+ is on youtube
However, edit /etc/slackpkg/slackpkgplus.conf
The file is big and man page is big too, but to start you need only to reach the statement MIRRORPLUS['...']=....
You can see a list of known repositories in /usr/doc/slackpkg+-1.8.0/repositories.lst
Uncommenting the row already present in default configuration is a good start
You can see a larger list of repositories in
https://slakfinder.org/showrepo.php
If you have some problem or you want to report bug or ask for improvement, the best place is the apposite thread
https://www.linuxquestions.org/quest...-a-4175669481/
you may need to run 'slackpkg update gpg' everytime you add/modify a repository