Is it permissible to specify the manner of death when supplicating, such as saying: "O Lord, take my soul while I am prostrating to You in Ramadan, or while I am performing Hajj," or is this matter predetermined and unchangeable?

There is no harm in supplicating for a servant to die while engaged in a righteous deed or in a blessed place, as this is an indication of a good ending and is permissible, similar to the supplication of 'Umar ibn al-Khattab, may Allah be pleased with him: "O Allah, grant me martyrdom in Your path, and make my death in the city of Your Messenger." And the Messenger of Allah, peace and blessings be upon him, said: "When Allah wills good for a servant, He utilizes him," meaning He enables him to perform righteous deeds before death. And seeking a good ending and asking for forgiveness are among the greatest needs. Supplication does not contradict divine decree, for it is also part of Allah's decree.