The proper URL appears to be https://localhost:8443/osapi/v1/namespace/<projectname>/buildConfigs/<buildconfigname>/instantiate with payload:

      var req = {
        kind: "BuildRequest",
        apiVersion: "v1beta3",
        metadata: {
          name: buildConfigName
        }
      }; 

Thought is to:

1. Update UrlBuilder to handle subresource
2. Update IClient to take an arbitrary subresource for a post (additional create with another flag?)
3. Add a capability to BuildConfigs