zendesk.com

In Zendesk Admin Center, go to Apps and integrations → APIs → Zendesk API and create a token. Zendesk documents API tokens in Security and authentication. When using it against Zendesk REST APIs, send it with your Zendesk account email via HTTP Basic auth as `{email}/token:{api_token}`.

Create an OAuth client in your Zendesk instance and complete a Zendesk OAuth flow as described in Security and authentication and creating and using OAuth tokens with the API. Use the resulting access token as a bearer token.

Sign in to Sell, open Integrations → OAuth, then in Access Tokens click + Add User Token to mint a personal access token, as shown in Make your first call. Copy the token when shown; after closing the modal it cannot be retrieved again. Send it as `Authorization: Bearer {token}`.

Register an application in Sell at app.futuresimple.com/settings/oauth/apps to get a client_id and client_secret, then complete one of Sell's OAuth 2.0 flows to obtain an access token, as documented in OAuth 2.0 Introduction. Use the access token as `Authorization: Bearer {access_token}`.

Create a Zendesk Chat OAuth application and complete the Chat OAuth flow described in OAuth Authentication. Use the resulting access token as `Authorization: Bearer {access_token}` for Chat API requests.