[AdamsLT]

Just check if the variable siren is 1 or 0 (return an error message if not).

Then pass the variable to CreateVehicle at the end like in the wiki.

https://sampwiki.blast.hk/wiki/CreateVehicle
example:
CreateVehicle(model, x, y, z, rotation, color1, color2, respawn_time, siren);

Edit: you provided the code as I posted, so I can see that you forgot to add respawn_time in the middle. Instead of respawn_time you can use -1 (like it says in the wiki) and it will never respawn and then pass siren, it should work.

I advise saving rotation data along with coordinates for the car so that it doesn't spawn unpredictably.