For testing COSMIC desktop applet, you can build it with help of Makefile.
git clone https://github.com/hepp3n/kdeconnect.git
cd kdeconnect
make build
sudo make install
sudo make uninstall
Make sure you have rustup.rs installed on your system.
You might need also libxkbcommon-dev dependency. If it won't build, please create an issue.
You can also build this applet as flatpak package. You need to install flatpak-builder and then run this command:
flatpak-builder --force-clean --user --install-deps-from=flathub --repo=repo --install builddir dev.heppen.CosmicExtConnect.json