Can't Find Various DDS formats

Hello
I’ve been trying to export DDS files in different formats one being. “8.8.8.8 ARGB, 32-bit unsigned sub-format” and the other being “1.5.5.5 ARGB 16-bit unsigned variant” which a wiki told me were the correct ones for the thing I’m doing. I don’t have any 1.5.5.5 ARGB option I do have an 8.8.8.8 BGRA option which I think is the same but very unsure. Is there something I need to install to get 1.5.5.5 ARGB or does it simply not exist?