There are specific supplications mentioned by the prophet (ﷺ), although the time of opening the fast is a blessed time when lots of du’as should be made.
The Prophet (ﷺ) said: “Three people will not have their supplications rejected: a fasting person until he breaks his fast, a just ruler, and an oppressed person.” [Sunan of Ibn Majah Hadith 3.1752, Jami’ Tirmidhi Hadith 2248]
There are some supplications that are specifically mentioned by the prophet (ﷺ)
ذَهَبَ الظَّمَأُ وَ ابْتَلَّتِ الْعُرُوقُ، وَ ثَبَتَ الأجْرُ إنْ شَاءَ اللَّهُ
Dhahaba al-zama’u, wa abtallat il-‘urooqu, wa thabata al-ajr insha Allah
The thirst is gone, the veins are moistened and the reward is confirmed, if Allah wills.
Marwan ibn Salim al-Muqaffa’ said: I saw Ibn Umar holding his bread with his hand and cutting what exceeded the handful of it. He (Ibn Umar) told that the Prophet (ﷺ) said when he broke his fast: Thirst has gone, the arteries are moist, and the reward is sure, if Allah wills. (Dhahabazh-zhumma’u wabtallatil ‘urooqu wa thabatal ajru inshallah) [Sunan of Abu Dawood Hadith 2.2350]
This can also be said after breaking the fast.
There is another du’a that can be said, but it has not as authentically reported from the prophet (ﷺ).
Mu’adh ibn Zuhrah reported that the Prophet of Allah (ﷺ) used to say when he broke his fast: “O Allah, for You I have fasted, and with Your provision I have broken my fast.” [Sunan of Abu Dawood Hadith 2.2351] (this hadeeth has been reported in mursal form and thus is considered weaker.)
اللَّهُمَّ اِنِّى لَكَ صُمْتُ وَبِكَ امنْتُ وَ عَلى رِزْقِكَ اَفْطَرْتُ
Allahumma inni laka sumtu wa bika aamantu wa ‘ala rizq-ika aftartu