Download the public key and put it in
/etc/apt/keyrings/meshtastic-deb.gpg. You can achieve this with:
wget -qO- https://meshtastic.github.io/meshtastic-deb.asc | sudo tee /etc/apt/keyrings/meshtastic-deb.asc >/dev/null
Next, create the source in /etc/apt/sources.list.d/
echo "deb [arch=all signed-by=/etc/apt/keyrings/meshtastic-deb.asc] https://meshtastic.github.io/deb stable main" | sudo tee /etc/apt/sources.list.d/meshtastic-deb.list >/dev/null
If you want to have the latest version of the software, you can replace stable with unstable.
Then run apt update && apt install -y meshtasticd.