How to make a 3rd level feature like Whispers of the Dead for a Homebrewed Subclass

Hello,

I'm trying to make a homebrewed Artificer and I'd like to implement a feat at 3rd level like the Phantom Rouge's Whispers of the Dead: 

When you choose this archetype at 3rd level, echoes of those who have died cling to you. Whenever you finish a short or long rest, you can gain one skill or tool proficiency of your choice, as a ghostly presence shares its knowledge with you. You lose this proficiency when you use this feature to choose a different proficiency that you lack.

Does anyone know how to make this possible using D&D Beyond's Homebrew subclass tools? Ideally, it would automatically reset after a short/long rest and it would show the temporary skill or tool proficiency on my character sheet while playing. Any tips help! Thank you so much!!