How to add immunity to a specific condition

I'm working on a homebrew race and I want to give them the trait of not having to roll for exhaustion caused by extreme cold/blizzard conditions because of its origin but I don't know how to specify that, and can only find the option to do general immunity from exhaustion

Complete immunity to exhaustion is way too overpowered. I would say just give it advantage on constitution saving throws or something similar.

As for immunity on effects caused by extreme cold, you can just specify that as a Trait. There's not really anything to add to modifiers or saving throws, so it's just freeform text. If you're struggling with the text itself, it doesn't have to be anything fancier than "You are accustomed to harsh weather conditions. You do not suffer effects caused by extreme cold."