Now that is an interesting question. Used to be via ickStream, but you can now directly link to Tidal.
I followed the instructions here https://support.tidal.com/hc/en-us/articles/201871191-Squeezebox . After faffing around registering this, that and the other I can either access Tidal via ickStream, or directly. Basically I have no real idea what the ins and outs are - I just followed the recipe. All of this working under iPeng http://penguinlovesmusic.de/ because that gives me an intuitive and easy to use interface to the NAS, to the Transporter, and Tidal. I've also got the TV connected to the Transporter via optical, and a CD transport via SPDIF.
I fiddled around with external DAC's connected to the Transporter, but just ended up using the inboard DAC, because it is a bit good - even fed via Jung Superregulators.
Getting it all working is a bit of a fight first time. There are people on forums (Stereonet) who haven't managed to get a Transporter working satisfactorily - like I said, nothing like plug and play!