How do I select Sorcerer Signature Spells?

[Hammer_Of_The_Circle]

I cannot figure out how to add signature spells.

On the character sheet:

The Classes > Sorcerer > Class Features > Signature Spells entry provides only a description.

None of the spells are interact-able with a toggle to make them signature.

The Permanent Added Abilities > Signature Spells (Add) doesn't present an option to select one of your spells or, as far as I can tell, make eligible the selection of spells in another spot in the character sheet.

How do I select Sorcerer Signature Spells?

Thanks!