Mouse and keyboard sharing

I'd like to share mouse and keyboard with a machine running Parabola.

Synergy is in Trisquel repository but as far as I am aware, it is no more free software. Barrier is a free software equivalent but there is no package for Trisquel (yet). In Parabola, there is barrier available but not synergy.

Barrier is not available with Guix. I checked the Debian barrier package, it has unsatisfied dependencies because of Qt. https://github.com/debauchee/barrier says there is flatpack and there is a link to https://github.com/flathub/com.github.debauchee.barrier but I never used any such thing and I am struggling to understand how to use that.

I installed the flatpak Trisquel package package and I searched for advice. I found some ubuntu help saying to run 'sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo' but I was hoping to refer only to barrier, not to this repository which contains I don't know what.

After reading of the man page flatpak-remote-add , I tried this:

$ flatpak remote-add flathub https://github.com
error: GPG verification enabled, but no summary found (check that the configured URL in remote config is correct)
$ 

Then I don't really now what to do.