In general, attractions become available for booking
DAS Return Times in the park once the attraction has a posted wait time. That usually does happen once the park opens for resort guest early entry. Not all attractions open for early open (and some may even open later than the general park opening time).
Attractions will show up under the DAS tile on the My Disney Experience as ‘DAS Eligible Options’ and also by tapping on the individual attractions from the map or the attraction list.
The person registered for DAS and all linked people you want to add to the DAS Return Time need to have entered the park to add them.