When should I recite dhikr أمسينا وأمسى الملك لله...رب أسألك خير ما في هذه الليلة ...before maghrib or after maghrib?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and Messenger.

The Thikr (Allah's remembrance) that you have mentioned in the question is from the evening and morning Thikr. It is better for the evening Thikr to be recited between ‘Asr and Maghrib, as it is better for the morning Thikr to be recited between the Fajr prayer and Sunrise. Allah Says (what means): {And exalt Him morning and afternoon.} [Quran 33:42] Morning is the beginning of the day, and afternoon is the time between ‘Asr to Maghrib.

Allah also Says (what means): {… and exalt [Allah] with praise of your Lord in the evening and the morning.} [Quran40:55]

Morning is the beginning of the day, and evening is the end of the day. Allah further Says (what means): {… and exalt [Allah] with praise of your Lord before the rising of the sun and before its setting.} [Quran50:39] Ibn Alqayyim  says in his book Alwabil As-sayyib: “{… and exalt [Allah] with praise of your Lord before the rising of the sun and before its setting.} is the explanation of what has been mentioned in the Hadith: “Whoever says such and such in the morning and in the evening; what is meant by it is before sunrise and before sunset, and this Thikr takes place after Fajr and after ‘Asr prayers.

On the other hand, whoever has missed this time, he can recite the morning Thikr after sunrise, and recite the evening Thikr after sunset.

Allah knows best.