assign HTTPS to node.js socket.io

i want to create HTTPS server to my node.js socket.io server, did it with self sign certification, using this code

    var fs = require( 'fs' );
var app = require('express')();
var https        = require('https');
var server = https.createServer({
    key: fs.readFileSync('C:/ssl/ia.key'),
    cert: fs.readFileSync('C:/ssl/ia.crt'),
    requestCert: false,
    rejectUnauthorized: false
},app);
server.listen(8888);

but when purchasing real one i only get .crt file, how to secure my node app using it on windows server?

via DevMohammad