title: IdentityServer 4: how can I add a custom external authentication?
body: <p>I'm using IdentityServer4 and I would like to add a custom external provider.
  in the doc, I saw that I can use AddGoogle, AddGitHut etc
  but there is no AddCustom.
  I would like to use an old legacy system, based on the CAS protocol.</p>
  
  <p>many thanks
  Pierre</p>