As the title says, I'm struggling to figure out how to add a spell in my homebrew collection to the additional specific spells in a subclass I'm homebrewing. I type in the name of the homebrew spell as I would the name of a normal spell, but it won't show up?

It should be in the Additional Specific Spells section of the Basic Information. Is your spell in your homebrew collection or just in your creations?

The spell is in my homebrew collection, and still doesn't seem to be showing up when I type its name into thee correct section?

Can you post the URL of the spell and your homebrew subclass and a mod can take a look hopefully. It is fine if they are private homebrew.

Is it one of your homebrews or a homebrew spell you’ve added to your collection from someone else?

A homebrew spell I've added to my collection from someone else

You can only use your own homebrew spell creations when creating a homebrew subclass/item/feat/etc.