Is there a specific prescribed dhikr in the prostrations of forgetfulness, or is one free to choose?

Answer

In the name of Allah, it has been narrated from the Infallibles (peace be upon them) to say: "Bismillāh wa billāh, as-salāmu ‘alayka ayyuhā al-nabiyyu wa raḥmatullāhi wa barakātuh." Allah knows best.