Like the title says, I am working on a subclass and I am trying to work out a feature that makes your unarmored AC equal to 10 + Con + Proficiency bonus. I've tried many things, and so far was able to ignore the unarmored dex bonus to AC and add the CON bonus. But I can't seem to get the proficiency bonus worked in. Right now I've tried using a Modifier of Bonus - Unarmored Armor Class, with a fixed value of 0 and an additional bonus of Proficiency Bonus and that does not seem to be working. I've also tried just adding the additional proficiency bonus to the modifier Bonus - Unarmored Armor Class Primary Stat CON which hasn't worked. Any help would be amazing!

Try:

Modifier: Set->Subtype: Unarmored Armor Class->Ability: Con->Additional Bonus Type: Proficiency

After you add anything to, change anything on, or remove anything from any homebrew, you MUST re-save the entire homebrew using the main [SAVE CHANGES] button under Basic Information or the system won’t pick up your changes. After that it can sometimes take up to 20 minutes for those changes to push through to the character builder/sheet.

This worked perfectly, thanks!