Unfortunately you can't send SMS from Signal in the iOS version of the app. And yes, I also want to be able to send an SMS from my desktop through my phone (like android messaging or imessage).

https://support.signal.org/hc/en-us/articles/360007321171-Ca...

Sure, iOS doesn't allow you to intercept SMS but others have already figured out how to work around this by assigning a virtual phone number.