I suspect it probably doesn't matter much whether you enter the OAT or ATIS temperature as they'd be close enough not to make much difference.
While I agree it wouldn't make much difference ... the more accurate one for the airmass calculations would be the ATIS temp. That's the one taken from inside a Stevenson screen/instrument shelter which shields the temperature reading from direct heat radiation from outside sources. Your OAT guage, sitting in the open above that hot tarmac (or covered in blowing snow or whatever) doesn't have that advantage.
I would suspect that's why the box doesn't just scrape the OAT reading, and instead asks you to type it in.