Is it permissible to supplicate with the words: "O Allah, choose for me and do not give me a choice" in specific matters when one is indecisive and unable to perform the Istikharah prayer due to not being resolute about a particular matter?

It is permissible to make the supplications mentioned both during and outside of prayer. Both a person performing prayer and one who is not may supplicate for the good of this world and the Hereafter. Istikhara (seeking guidance from Allah) is performed for matters where the correct course of action is not clear, whereby the servant entrusts his affair to Allah so that He may choose what is best for him. If there is no specific matter for which to perform Istikhara, one should pray to Allah to open up for him what contains goodness and benefit.