Is the prostration of forgetfulness (sujud al-sahw) obligatory for someone who inadvertently recited the qunut supplication after the second raka'ah of the Maghrib prayer, and what is the ruling if they performed it?

Qunut is performed in the last rak'ah of the prayer. So, if one forgets and performs qunut in the second rak'ah, their prayer is valid, and they should perform the prostration of forgetfulness (sujud as-sahw). It is recommended that the prostration of forgetfulness be performed after the taslim (salutation of peace).