I've done all these without any errors:
$ sudo dpkg --add-architecture i386
$ wget -nc https://dl.winehq.org/wine-builds/Release.key
$ sudo apt-key add Release.key
$ gksu gedit /etc/apt/sources.list
deb https://dl.winehq.org/wine-builds/debian/ stretch main
deb http://http.debian.net/debian stretch-backports main
$ sudo apt-get update
However when I'm trying to:
$ sudo apt-get install --install-recommends winehq-staging
It says unable to locate winehq-stable. It also says the same to winehq-stable, wine-stable and other various combinations. Typing only wine worked, but it was version 1.8.7-2 (DXVK requires 3.10 or newer) so I purged it. Where do I get the correct package?