This is not recommended on stable branch as it requires custom packages and therefore entirely unsupported. The same goes for third party repos - they are unsupported.
- [root tip] [How To] Update Manjaro the smart way
- [Need-To-Know] About Manjaro and AUR
- Unofficial user repositories - ArchWiki
The custom script repo is Arch User Repository - and as the name implies - assumes Arch package level - which is only available when using unstable branch.
If you come to a point where this becomes important only 2 steps is required
- Switch to unstable branch and run a full system sync
sudo pacman-mirrors -aS unstable && sudo pacman -Syyu - Open the package manager and navigate to Preferences → Third Party tab and enable AUR.
NOTE: Do not enable neither update check options - this may cause issues when doing regular sync where packages has been removed from the repo. This removal is often done upstream from Arch - and in such case the packages is moved to AUR - which in turn causes sync issues.