(client/request {:method :get
                 :url "http://localhost:18080/get"
                 :http-client-context my-http-client-context
                 :http-request-config my-request-config})

;; Our code looks like this...
(defn additional-cljhttp-options [proxy-host proxy-port proxy-user proxy-pass]
  (let [host (HttpHost. proxy-host proxy-port "http")
        creds (doto (BasicCredentialsProvider.)
                (.setCredentials AuthScope/ANY (UsernamePasswordCredentials. proxy-user proxy-pass)))
        basic-scheme (doto (BasicScheme.)
                       (.processChallenge (BasicHeader. HttpHeaders/PROXY_AUTHENTICATE "Basic")))
        auth-cache (doto (BasicAuthCache.)
                     (.put host basic-scheme))
        context (doto (HttpClientContext/create)
                  (.setCredentialsProvider creds)
                  (.setAuthCache auth-cache))
        request-config (-> (RequestConfig/custom) (.setProxy host) (.build))]
    {:http-client-context context :http-request-config request-config}))