if (client.connect(BIserver, 818)) { // (81) Port number of the Blue Iris web server //CHANGE THIS if you don't use port 81.
If you are going to have useless comments, they MUST be accurate useless comments.
client.print( "&trigger&user=");
The data in the GET request is usually in the form of name=value pairs, separated by &s. What is the value associated with trigger?
which I don't think works with the nodemcu
If the code compiles, WiFiClient is a class defined in the ESP8266WiFi header file, so it must be able to work with the ESP8266 device.
Are you able to connect to the camera server?