Custom code in Paginated reports

Hello,

I'm creating a report using Power BI Report Builder on Pro subscription.

I would like to use LookupSet function in a way described here: https://docs.microsoft.com/en-us/sql/reporting-services/report-design/report-builder-functions-lookupset-function?view=sql-server-ver15

So I need to add custom function and then use it in an expression. I can't find a place where I could add it. Is it only achievable using premium subscription?

Thanks,

Rafal