In the Colour Tab there’s a Colour Management section:
Tick Custom and click on (None). You are presented with a new window with a bunch of dcp profiles:
The problem is, I just noticed, that this specific profile is not part of the latest RawTherapee (I think I got it from ART). So you need to do the following first:
Download and place this file: SONY ILCE-7SM3.dcp (63.8 KB) into the directory shown in the above screenprint (on Linux it says: opt rawtherapee dcpprofiles as shown above. Not sure what it is on a mac).
Once that is done you might have to restart RawTherapee, not sure. If it shows up in the list, you can now select it.
Hope this is clear enough, if not: Don’t hesitate to ask.