This can be done after the appointment is scheduled. Follow the steps below:  

1. Go to my appointments tab
2. Filter by date at the top
3. Select the appointment 
4. Click on 'manage appointment'
5. Below the statuses, select either 'arriving early' or 'running late'
6. Type in the ETA
7. Click 'save'

As always, this is also available via the API if you'd like to build a custom integration with ELD providers.