Making a Custom Subclass

Hello. I wanted to go with a homebrew wizard arcane tradition, but it doesn't show a "custom" option in the traditions drop down when leveling up. There's is one when choosing an entire class itself, but not subclass. How do I add this in? https://i.redd.it/1deruk1kguw21.png

Just skip it, and add it in manually in the Traits & Features section of the character sheet. That's all the Custom selection does anyway.