Connector Appliance does not work with proxy configuration 
As soon as proxy config is added, connectivity check fails for all the URLs

Need to set proxy to use no-auth or basic auth (negotiate not supported at the moment)
https://docs.citrix.com/en-us/citrix-cloud/citrix-cloud-resource-locations/connector-appliance.html#network-requirements
This is what the document states:

• If you use a proxy with Connector Appliance, the proxy must be unauthenticated or use basic authentication.

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Problem Cause

Issue is due to the proxies being secured with kerberos auth, however the Connector Appliance only supports proxies which are unauthenticated or using basic auth. https://docs.citrix.com/en-us/citrix-cloud/citrix-cloud-resource-locations/connector-appliance.html#network-requirements