I propose the following keychain entry for macOS:
security add-generic-password \
-l 'auth_token' \
-a 'your beeminder username' \
-s 'www.beeminder.com' \
-j 'auth_token for use of the Beeminder API' \ # https://www.beeminder.com/api/v1/auth_token.json
-w 'here_goes_your_auth_token' \
-U
to be retrieved again with:
security find-generic-password \
-l 'auth_token' \
-a 'your beeminder username' \
-s 'www.beeminder.com' \
-w
which will write it to stdout.