I can't figure out how to add prerequisites to class feature options. When I do it just shows up as a bar.

You have to create a class feature with options, and set it up that the player can choose at least one of those options. Then you have to set up a second feature with different options, and set those options to be automatically granted, and set the options from the first feature as the prerequisites for the options to be granted from the second feature.

I went into more detail about it here:

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/67361-subclass-help#c4

I hope that helps.

Hi Sposta, I'm creating a Warlock sub-class and trying to establish a pre-requisite for an option, but nothing is populating in that field.

Specifically, I'm trying to make the option only available if the player has selected the "Pact of the Blade" as their Pact Boon, which comes from the Warlock base class (similar to how you can only take the "Improved Pact Weapon" invocation if you have chosen the Pact of the Blade). 

How do I go about this?

Only subclass feature options can be selected as prerequisites for other subclass feature options. We don’t have access to base class level stuff in the content builder.