Answered by Shaykh Amjad Rasheed
If one says the opening takbir [s. the saying “Allahu Akbar”] to pray dhuhr, then he remembers that he wanted to combine his intention with the prayer of istikhara, is his combined intention valid even though he already began the prayer and said the opening takbir?
If the person praying has already finished saying the opening takbir, then his intention to combine intentions is not valid, whether it is a the prayer of istikhara or another.
Amjad Rasheed
[Translated by Sr. Shazia Ahmad]