String url = "https://YOUR_TENANT_ID.infusionsoft.com/api/xmlrpc";
String xmls = "<?xml version=\"1.0\"?>\n" +
        "<methodCall>\n" +
        "    <methodName>ContactService.listLinkedContacts</methodName>\n" +
        "    <params>\n" +
        "        <param>\n" +
        "         <value><string>YOUR_LEGACY_KEY</string></value>\n" +
        "        </param>\n" +
        "        <param>\n" +
        "         <value><int>1</int></value>\n" +
        "        </param>\n" +
        "    </params>\n" +
        "</methodCall>";

URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();

con.setRequestMethod("POST");
con.setRequestProperty("Content-Type", "application/xml");

con.setDoOutput(true);
OutputStream os = con.getOutputStream();
os.write(xmls.getBytes());
os.flush();
os.close();

int responseCode = con.getResponseCode();
System.out.println("Response Code : " + responseCode);

Scanner s = new Scanner(con.getInputStream());
while (s.hasNext()) {
    System.out.println(s.nextLine());
}
s.close();