I'm trying to create a homebrew subclass with a feature that grants proficiency if you don't have proficiency, and applies another bonus if you do have proficiency. While researching how to implement this, I found some precedent in the Artificer Alchemist's Tool Proficiency (for reference: "You gain proficiency with alchemist’s supplies. If you already have this proficiency, you gain proficiency with one other type of artisan’s tools of your choice.") In the character builder, this is applied intelligently, where if my character already has proficiency with those tools, I'm given an option select under the feature; otherwise it automatically applies alchemist's supplies proficiency and I see no option select under the feature. When I tried to copy this implementation by creating a copy of the subclass, there didn't seem to be anything besides an option select modifier. Is this simply not available to homebrew subclasses yet?

I explained it in this thread:

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/67361-subclass-help?comment=4

I hope that helps.