DirectQuery: Decode text from Base64

direct query mode relies on querying the data source in realtime, and transformations like Base64 decoding happen within Power bi's internal engine, you can preprocess the data in etl to to decode then import it in powerbi. or you can go for source level decoding ie decode at database level

I think AnalyticPulse is right. Decoding isn't likely compatible with DirectQuery since it breaks query folding. The best option would be to decode it in the database before connecting.

Alternatively, it's likely possible to decode using DAX but you'd have to write your own custom logic to do that.