You can send SMS through the [Signal] app?! Been using it for a few weeks now but haven't figured this out. Or do you mean send SMS through the stock SMS app?
To do so, you have to go into your default apps, set the default messaging app to Signal (factory default is Messages for Pixel devices, likely whatever Samsung uses for Samsung devices). I just checked to see if there were any extra settings you have to enable in the Signal app, but it doesn't appear to be so. If a user uses both SMS or encrypted chat, you can long press the send button and decide between which it will use.
Do note, that any previous messages sent via SMS from other apps won't show up in Signal for message history. Likewise the same if/when you switch to another SMS app.