if you look at the content of the url - it is stated the following url is to be set for the mirrorlist
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add the following to your /etc/pacman.d/mirrorlist
Server = https://mirrors.manjaro.org/repo/BRANCH/$repo/$arch
Note: replace BRANCH with your branch you want to use
Possible options: stable, testing, unstable, arm-stable, arm-testing, arm-unstable
https://mirrors.manjaro.org/repo
As buildiso already contains the branch info - then the above will be how you set in your tools configuration