You can use Advanced formula fields in your service spaces like any other field by adding them to the Request form or Work item view of your Requests depending on wether you want to show them to your agents or customers.
You can use formula fields in your service spaces to enable your agents by showing them the data they need to fulfill the requests. You can even use formula fields to specificly provide your service customers tailored information in the portal view of their requests.
Configuration
To show advanced formula fields to your agents, create them as usual add them to the Work item view of the relevant Request types. To show them to your customers in the portal view, add them to the Request form instead.
Visibility for Customers
As Advanced formula fields are read-only fields, they will never be shown while a customer is creating a new Request, only if they view an existing request in the Service portal.