Trello REST API

Create or open an app in the Trello apps administration page, then open the app's API Key tab and choose Generate a new API Key. Trello's docs note that API keys are tied to a Trello app/Power-Up; if this is your first visit to the admin portal you may first need to complete the Joint Development Agreement as described in App Admin Portal.

First create a Trello API key in the Trello apps administration page. Then send the user through Trello's token authorization flow at https://trello.com/1/authorize as documented in Authorizing With Trello's REST API. After the user grants access, Trello returns a user token scoped to that API key. The docs also mention an alternative legacy Basic OAuth 1.0 flow, but the 1/authorize token flow is the primary path.