Here are the steps to follow to select AirPlay on the Hearthis.at app:
1) open the Hearthis.at app, select a playlist and click on the three dots (see picture)
2) select AirPlay (see picture)
3) select the speaker(s) from the list where to listen to (see picture)
Read also: How to follow Ramblin’ Tunes with the Hearthis.at app