Configuring a trackball with multiple buttons

Hi,
I bought a trackball (I think I need to watch out for RSI - a split keyboard is next, but quite expensive) with multiple buttons. In the default configuration, you cannot define those. I tried adding the buttons in the keyboard-shortcuts, but it doesn’t take the mouse-clicks, only keyboard.
Its a Bluetooth connected trackball (it has usb, but I don’t want to loose a port to the usb-dongle).
I found this Input Remapper and installed with BoxBuddy:

sudo dnf install input-remapper
sudo systemctl enable --now input-remapper

Then:

sudo systemctl enable --now input-remapper
Created symlink /etc/systemd/system/default.target.wants/input-remapper.service → /usr/lib/systemd/system/input-remapper.service.
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is inactief

So, how do I “boot the system with systemd as init system”?