[ame="https://www.youtube.com/watch?v=JYQE9sO-Lhc"]Subimods V2 Hella Horn Kit Installation 2015-2016 WRX / STI - YouTube[/ame]
go to the 3:30 mark. find the plug that looks like that and plug it in. the plug only goes in 1 way so I doubt you will get it wrong. the plug should be near the lower passenger side corner of the radiator if I recall correctly and also next to the temperature sensor.
I personally have not installed this harness, but I remember unplugging my horn down there when I was installing my oil cooler. it should be the plug you're looking for.
EDIT: the website you bought the harness from does not have the BRZ under the compatibility list. from the pictures, it seems as the harness you bought is different from the one Subie mods offers for the BRZ and 2015+ WRX since they both share the same horn plug (the compatibility for the harness you linked stops at 2014 WRX). this is the harness you want:
https://subimods.com/subimods-v2-hel...m-h-hv2va.html