I’ve been manually fixing this in my emails and texts for years now, and finally decided to google it today.
To get a line feed in your text when using SIRI, simply say, “New Line”.
There are lots other really useful dictation commands you can use with Siri. Here are a few that are particularly helpful:
|new line||starts a new line of text|
|new paragraph||starts a new paragraph|
|all caps on/off||turns caps loc on/off|
Hope those are helpful!