In your Slack browser tab, open DevTools (F12):

1. Token — Console, run: JSON.parse(localStorage.localConfig_v2).teams, expand your workspace, copy its token value.
2. d cookie — Application ▸ Cookies ▸ https://app.slack.com, copy the value of the d row.

Kept only in this browser's local storage — nothing leaves your machine except calls to Slack.