Check Header on outbound

Hello.

I'm attempting to do a header verification within NGINX and I can't seem to find a good place to start.

I'm running NGINX 1.14.0 on Ubuntu 18.04 with default settings. My flow begins from inside our private network:

MyApp -> Proxy(Check Header) -> Outside Service.

In the case that the header doesn't match the definition, it should kick back a 403 to 'MyApp'.

I feel like I'm over-complicating this. Can someone point me in the right direction on where to start?

Thanks..

Best,

Jason M.