Resource Proxy With PKI

I am attempting to use the Esri resource-proxy to proxy all WMS requests to other servers. In doing so I am attempting to get around a CORs issue that does not require a user browser plugin. I can get the proxy working for any site that does not require user authentication with a PKI certificate, but when I attempt to access a site that requires a cert, the request fails and throws a "PKIX path building failed" error. It appears as though the user’s certificate is not being passed from the proxy to the end site.

Is there a way to pass the user’s PKI credential through the proxy to the desired destination?

I currently have a Tomcat instance configured to use PKI for user authentication (this part is working) on my local machine, but am not having success passing that credential to the end site.

I have noticed the proxy config can be set to use different authentication methods (access token, username/password, etc..) but all of these seem to be preconfigured and not dynamic per user.

I am not too familiar with PKI and fairly new to web development.