Is it possible to add basic auth to services: like mailhog or jenkins?

scheduler:
    engine: jenkins
    endpoints:
      <http://scheduler.xxx|scheduler.xxx>:
          <<: *frontend-auth

seems to be ignored but

groups:
  DE:
    region: EU
    applications:
      Zed:
        application: zed
        endpoints:
          os.xxxx:
            store: DE
            <<: *frontend-auth

is working fine Or can i add credentials login for jenkins, mailhog and dashboard?

I believe this is conceptually wrong and these services should not be accessible from public networks.

Its only in internal network available and it is secured with a https reverse proxy

At the moment those URLs (jenkins, other services) are not propagated into Spryker Cloud. DevOps should give you the proper ones.