Homebrew campaign, acquired a "Runic Robe" which allows the wearer to roll a d4 after every long rest, 

if the roll is a 1 or 2 wearer gets 1 additional prepared spell in the list

if the roll is a 3 or 4 wearer gets 2 additional prepared spells in the list

How do I set this up in D&D Beyond ?

When you click "manage spells" you can expand the spell description and click "add spell" at the bottom. This bypasses the normal cap.

Thank you !

I have a Druid and can't find this option. I would like to learn an extra Cantrip.

Quote from Ryex >>

I have a Druid and can't find this option. I would like to learn an extra Cantrip.

 Doesn't work on cantrips, you will have to make a homebrew feat.