How can I get user custom attribute value in property mapping?

[ser]

My users have set a custom attribute 'upn'

How can I get this attribute in property mapping? The example property mapping for proxy is:

return {
    "ak_proxy": {
        "user_attributes": {
            "additionalHeaders": {
                "X-App-User": request.user.username
            }
        }
    }
}

... and I really do not get how can I call that upn attribute instead of request.user.username in above example. I tried user.attributes.upn but Mr Authentik says there is no such thing.

Please help!