Members who wish to make use of iRacing's /data API web endpoint will need to request OAuth Client Credentials. Instructions for doing this are found at Client Registration. Most members will be looking for a Password Limited client. The Authorization Code is only needed for members developing applications which other users will log in to, and the application will then make requests on behalf of the logged in user.
Complete documentation of the OAuth setup can be found at Introduction - iRacing.com Auth Service.
Discussion of the /data API can be found at:
Note that the /data API endpoint is NOT the same API as used by the sim for local telemetry logging, also referred to as the iRacing SDK. Discussion on the sim API can be found at: