I had the same experience. I entered the first name in the first box and the rest of the full name in the second box.
after my passport was issued, I asked to change my name according to the name on the passport document,
which is the ID that I will use to take the CMA exam.
I contacted the IMA institute to change my name according to my passport, and it responded within a week that my name had been changed and that the new name would be arranged as follows: first name, then middle name, then last name.
------------------------------
MOHAMED ELASHMAWY
Director/Manager
GIZA
Egypt
------------------------------