Hi @ssastrog,
This feature is only available in the mobile app. You can download this in the App Store for IOS and Google Play store if Android. See the screenshots for the apps below:
After downloading the application, you can add the trip purpose by clicking the Create Rule.
In the future,if you wanted to download the mobile app for QuickBooks, refer to this link: How to download the QuickBooks Online mobile app.
If you have any other concerns, keep me posted by replying to the thread. Have a good day.