Check in iReal Pro Settings for the global Tuning, options between A = 415 to 440 to 466 Hz. 
Normal is A = 440.

Use a chord as reference

1. create a new song with A5 as a chord (or similar);
2. set the song's default key as A for song info, tap/click Set;
3. make sure there is a barline on either side;
4. (optional, repeat the chord in the following bars);
5. set Jazz - Long notes as the playback style in Song info;
6. in Player controls, set tempo to 40

Transposing instrument: iReal Pro always plays in Concert key, choose the key of A in the Player controls as you would do normally for any song.

Use Keynote

The Keynote is a reference note normally used for singers to then find the starting note for a song. It plays the root note of the key a song is in instead of the count in sounds.

1. choose any song, or construct one as above;
2. set to the key of A in Player controls;
3. tempo to 40;
4. set Count in to 2 bars;
5. Set Count in Click 1 to Keynote
6. press Play

An "A" Keynote will play for the count in (Press Stop before the song starts playing)