For ProtonVPN as an example: you can go to the downloads section and download the wireguard config you want and then import it from the wireguard gui.
For me, I would never use other inferior VPN clients after knowing how efficient, supported and developed are the official Wireguard gui are.
Other providers who provide configs:
Yes that was what I was referring to. I suspect it was meant as some kind of marketing stunt. There are some interesting comments about this on one of the Dutch subreddits.
That link is broken for me.
Fixed the link