Any service can use the Sonos API to add themselves to the Sonos app.  The service themselves would have to initiate this with Sonos.  

Thanks so much for the answer! So, I will write to the app that I would like to hear on Sonos, rather than Sonos themselves, good to know.

If I’m not mistaken, it’s already on Sonos. Yep, right there, when I look under “Music & Content” under Services & Voice. 

i suppose it may be dependent on location. My account is in the US. 

Thanks @Airgetlam for the reply!

I checked the US version of services, but couldn’t find it on Sonos page (showing screenshot here) - are you sure it is the Waking Up app? 

Send them to https://developer.sonos.com/ for details of how to add their service to Sonos.

Apologies. I misread your post, and thought we were talking about Calm, not ‘Waking Up’. Follow the suggestion made by @controlav and @jgatie . ‘Waking Up’ just needs to add themselves to the Sonos ecosystem.

Thank you all for the suggestions! I wrote to the Waking Up, hope they’ll add the feature soon :)