In the identity server samples we find code like this in Startup.cs
var certFile = env.ApplicationBasePath + \"\\\\idsrv3test.pfx\";
var signingCer
Get a dedicated cert - either via your PKI or self-generate one:
http://brockallen.com/2015/06/01/makecert-and-creating-ssl-or-signing-certificates/
Import the key pair into the Windows certificate store, and load it from there at runtime.
To step up security, some people deploy the keys to a dedicated device (called an HSM) or to a dedicated machine (e.g. behind a firewall). The ITokenSigningService allows moving the actual token signing to that separate machine.