The 404 seems to be related to the fact that the request isn't actually making it to Splunk. By default, browsers don't allow cross-site scripting - Same origin policy. You will need to tell your web server to proxy specific requests to Splunk. For example, when running apache, following 2 lines of code do the trick.
SSLProxyEngine On
ProxyPass /proxy/ https://localhost:8089/
Hope this helps.
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