I want to create a martial tradition subclass where one of the features offers a choice of one of two proficiences (Investigation or Perception). Similar to how the Arcane Archer subclass offers that sort of proficiency choice with its Arcane Lore feature.

I can't figure out how to do it and was wondering if anyone here could point the way for me?

Give this "Astute Warrior" feature 2 Options: Investigation and Perception. In the first option, add the modifier Proficiency - Investigation, and in the second, add the modifier Proficiency - Perception.

Check the Has Options box on the feature itself.

Thanks! I tried that initially, but I must have done something wrong - will redo it and see if it works.. Thanks again. :)

Remember to Save Changes for each feature, and also at the main subclass edit page. Then it can take up to 20 minutes for homebrew changes to percolate to character sheets.