I'm creating a subclass in the DnD Homebrew Creations and part of an optional ability is to add a bonus based on the Charisma modifier. The description wording is shown below;

' you gain Darkvision with a range of 30 feet and a bonus to your passive Wisdom (Perception) score and to your Wisdom (Perception) checks. The bonus equals your Charisma modifier.'

I'm not sure how to do this within the 'Add a Modifer' feature in the homebrew page. It seems I can only add a fixed value but if the Charisma modifier changes in anyway, it means the homebrew has to change. Is there anyway to do this and I've missed something, or is it not possible currently within Homebrew Creation? 

Add A Modifer

Type: Bonus
Sub-Type: Perception Checks
Ability Score: CHA

Result: adds Charisma Modifier to Perception check modifier. Due to how Passive Perception works (10 + Perception Modifier) this will also increase Passive Perception by the Charisma modifier as well.

Add A Modifier

Type: Bonus
Sub-Type: Passive Perception
Ability Score: CHA

Result: adds Charisma Modifier to the Passive Perception only. If used with the above modifier then the Charisma modifier is effectively added to Passive Perception twice.

Add A Modifier
Type: Sense
Sub-Type: Darkvision
Fixed Value: 30

Result: Adds 30 ft Darkvision.