In order to register a client with Patreon, you'll need to be a creator first. You can get started as a creator if you have yet to here.
Once your page and tiers are setup, you may continue on to create an API client: https://www.patreon.com/portal/registration/register-clients
Click Create Client, and you'll want to be sure to fill out any required info and set the Client API Version to V2. You'll also need to provide a redirect URL; which https://0.0.0.0 should work just fine as in this case it won't be used, the field is just required.
Now you may click Create Client, and your Patreon API Client should be created. You can now go ahead and copy your Creator's Access Token, and add it to your Packix account within your developer settings.