According to Apple App distribution guide:
Use iTunes Connect to set a date when the app is available on the store. For example, you can choose a date that immediately releases the app to the store after it’s approved, or you can set a later date. Using a later availability date allows you to arrange other marketing activities around the launch of your app.
The screenshot in documentation is outdated though. See the image below.
Steps:
Login to iTunes connect
Select Apps
Select your application
Click Prepare for Submission
Scroll down the detail page to bottom to see option to manually
release the app.
Note that if you set a date too early, it is entirely possible that your app will not be released until and unless the review is complete.
Also, if you decide to update your app during review time, you will have to restart entire process and lose progress on current app review.