Install git-peek

You'll need to have git-peek installed for this to work.

brew install jarred-sumner/git-peek/git-peek

Make the Peek button work

Register the git-peek:// URL handler:

git peek -r

Then, head over to the test page:

To use Peek with a private repository, set $GITHUB_TOKEN in ~/.git-peek:

GITHUB_TOKEN="*******"
EDITOR="code" # optional

If you're having trouble with this, here is more text to read.