AirPlay is a system control on iPhone and iPad, not a setting inside iReal Pro. Open Control Center, press and hold the audio card at the top right, and choose your receiver.

 

Because iOS cannot tell an old AirPlay receiver from a new one, the wake delay that wakes Bluetooth speakers applies to AirPlay outputs too. Find it under Settings > Wireless Wake Delay, and set it to 0 if you would rather not have the short delay before each song.

 

For the full guide, see Sound output and audio devices on the iReal Pro website.