It is a bit more complicated than it could have been, due to how 
Blue iris splits up all the parts of the RTSP URL.
Dahua
Here is how I do it for Dahua, broken down Barney-style.
1) Copy the 
Main stream text into the 
Sub stream box.
2) Type a question mark (
?) after the text you pasted in the 
Sub stream box.
3) Copy the 
Params text into the 
Sub stream box after the question mark.
4) Change 
subtype=0 to 
subtype=1.  (or 
subtype=2 if using the second sub stream on a camera that has multiple)
View attachment 61281
Hikvision
My Hikvision cams don't use a Params section, so I just need to copy the 
Main Stream text and change 
{CAMNO} to 
2.
View attachment 61282
A similar procedure can be followed for most cameras.  You just need to figure out which part tells the camera which stream you want.  In some cases, there might not be a number or anything obvious, so you should google for the correct RTSP url format.  ispyconnect often has a good list.  Ex: "
ispyconnect hikvision rtsp URL"