Start with defining the event you want to track in your web instance. For example, if you have a live chat installed in your app and you want to trigger a survey after a customer closes a chat, then you'd have to track the "close chat" event.
Next, log in to your Retently account and create a new in-app survey template (or edit an existing one). Learn how to enable the in-app channel.
Then go to the Settings tab and make sure that the survey is set to trigger immediately or at leas in 1 second.
After you've done that you'll have to access the Setup tab and copy the tracking script. Make sure that the in-app tracking script runs whenever the "close chat" event is triggered.
Finally, you'll have to set up a new campaign in Retently and create a new Rule for an NPS campaign. Choose the in-app survey template you have just created and set 0 days for the new customers schedule. Optionally, you can disable the recurring surveys option to make sure that your customers will now receive further in-app surveys on a regular basis.