Can, register first for the subject first. You will be allowed
to go. I think it is given as off, so I dun think will waste any
leave (but depend on ur unit).
But remember the dates. Dun like me, register liao, take Oral liao,
then totally forget the exam dates (I was in BMT Tekong). Then some
of my friends have request off from my OC to take the exam then, I
was like... 
SH*T! I
forget about it! Hiaz....but then too late liao.....