Sorry mate, I just seen this. So you want the horn to go off at the same time as the siren?
The purpose of a relay is to allow the use of a low current signal to switch a higher current load.
I don't know what load the output on the pink wire can handle, so to be on the safe side I would wire it like this.
If it was a relay from a BMW, the coil connections would be 85 and 86 and the contact connections would be 30 and 87 as shown. But they will differ depending on what relay you use. As long as you get the connections to the coil and the contacts correct you'll be ok. Most relays will have a little diagram on the side to show which pins are the coil etc.