I need someone to help me figure out a way or a script that can get all reolink or any onvif cameras to fire their sirens/alarms all at the same time with the press of a button via blue iris.

Powershell cmd triggering an AI enabled Reolink camera Siren/AudioAlarm:

Invoke-WebRequest -Method Post -Uri 'http://<ipaddress>/cgi-bin/api.cgi?user=admin&password=<password>' -UseBasicParsing -Body '[{"cmd":"AudioAlarmPlay","action": 0, "param": {"alarm_mode": "times", "manual_switch": 1, "times": 1, "channel": 0}}]'

Save this to a .ps1 file, and can be run from camera Alert / On alert... / Run a program (or anywhere):

Program = powershell.exe
Parameters = -f <full path>\<filename.ps1> like -f C:\BlueIris\Scripts\PlayAudioAlarm.ps1

IMPORTANT:
Powershell cmd getting Reolink camera AI configuration; required for triggering Siren/AudioAlarm from a web request:

Invoke-WebRequest -Method Post -Uri 'http://<ipaddress>/cgi-bin/api.cgi?cmd=GetAICfg&user=admin&password=<password>'

Wouldn't that trigger only one camera?

Sync any additional cameras to the main trigger camera and/or create a group in Blue Iris specifically for the purpose as well.

The Reolink 811A would probably be a good choice for both its siren and its floodlight.