The Sitecore CDP OAuth2 implementation is not currently working.
Create a new Connected App in Salesforce. Find the App Manager in setup, and click New Connected App at top right. Enable OAuth settings and set the Callback URL to “https://oauth.pstmn.io/v1/callback” to allow testing interactive flows with Postman, or whatever value you require (not used by Sitecore CDP). Select appropriate scopes, e.g. api, refresh_token. Retrieve the Client ID and Secret once created.
Create a new Connection of type Destination in Sitecore CDP. Use OAuth2 authentication with the following settings:
Token URL: https://<salesforce.domain>/services/oauth2/token
Client ID: from Connected App
Client Secret: from Connected App
The Salesforce refresh_token or password flows can be used here. Click Show URL encoded form and add the required values.
Refresh token flow
grant_type: refresh_token
refresh_token: (perform authorization code flow with Postman or other client and copy the token)
Password flow
grant_type: password
username: (Salesforce username)
password: (Salesforce password)
The endpoint should be https://<salesforce.domain>/services/apexrest/cdp4sapi/guestdata.