https://github.com/Homebrew/homebrew-cask-versions/blob/master/Casks/visual-studio-code-insiders.rb
Run brew install homebrew/cask-versions/visual-studio-code-insiders
$ brew install homebrew/cask-versions/visual-studio-code-insiders
==> Tapping homebrew/cask-versions
Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask-versions'...
remote: Enumerating objects: 257301, done.
remote: Counting objects: 100% (22133/22133), done.
remote: Compressing objects: 100% (346/346), done.
remote: Total 257301 (delta 21896), reused 21914 (delta 21787), pack-reused 235168
Receiving objects: 100% (257301/257301), 65.40 MiB | 23.35 MiB/s, done.
Resolving deltas: 100% (178692/178692), done.
Tapped 230 casks (260 files, 72.7MB).
==> Downloading https://code.visualstudio.com/sha/download?build=insider&os=darwin
==> Downloading from https://az764295.vo.msecnd.net/insider/9084e081d4e89ed8ab67fce340d573c4e1378939/VSCode-darwin.zip
########################################################################################################################################################################################################### 100.0%
Warning: No checksum defined for cask 'visual-studio-code-insiders', skipping verification.
==> Installing Cask visual-studio-code-insiders
==> Moving App 'Visual Studio Code - Insiders.app' to '/Applications/Visual Studio Code - Insiders.app'
==> Linking Binary 'code' to '/usr/local/bin/code-insiders'
🍺 visual-studio-code-insiders was successfully installed!$ /usr/local/bin/code-insiders --version
1.79.0-insider
9084e081d4e89ed8ab67fce340d573c4e1378939
x64