Is supplicating with "O Allah, cause me to die and join me with the righteous" in prostration and in every prayer forbidden or disliked, and will it be accepted or not, and what is the evidence for each of these?

Supplicating for death is prohibited, unless one fears for their religion from tribulation. In Sahih Muslim, the Prophet, peace and blessings be upon him, said: "None of you should wish for death, nor pray for it before it comes to him. For when one of you dies, his deeds cease. And the believer's lifespan does not increase him except in goodness." As for wishing for martyrdom, it is not among the prohibited wishing for death; rather, it is encouraged. And the acceptance of supplications is from the knowledge of the Unseen.