How do I create a property I can use across sites connect to a hub?

There isn't a great option for you here unfortunately, especially since your requirement isn't just for pre-defined values. Some options below:

1. Utilize Enterprise Keywords that contains a collection of keywords you can use to tag your data and add new keywords dynamically. However, this isn't scoped to just your Hub (and associated sites). To use Enterprise Keywords in a List or Library, you would go into the List/Library Settings, click on Enterprise Metadata and Keywords Settings, then enable Enterprise Keywords. It will then appear as a column in that list/library. I don't really like this as it contains any keywords from anywhere across the entire tenant.
2. Create a Term Set in the Global Term Store and for the Submission Policy set it to Open (Users can add terms from a tagging application). This will allow you to use this Term Set in a Managed Metadata column and users will be able to use the pre-defined terms and add new ones (although not intuitively). Note however, that since this is a Term Set in the Global Term Store, then anyone else can use it for their sites too.