Prefer package from custom repo while building an ISO

Hi there,

I asked this question in the Matrix channel:

Is it possible to force a package to be installed from a custom repo rather than the manjaro community repo specified in the Packages-Desktop file while creating an ISO?
So e.g. I’ve set up a package mate-indicator-applet which is different from the package in the repositories and I want it to be installed on the ISO. However if I only add mate-indicator-applet in the Packages-Desktop it obviously prefers the manjaro community repo. How do I force to use the custom repo for this package?

I also got a nice answer that I should prepend my repo in the pacman.conf. So

this makes sense but I added a custom repo by putting a user-repos.conf file in the iso-profiles , so I cannot access the pacman.conf?

So the question remains how can I make the custom repo the preferred one while building an ISO?