First create a survey, then, when you are ready, press the "Copy Snippet" button. You will now have the snippet code ready to paste.
Go to your Freshdesk account and choose the "Admin" page from the left hand menu.
Choose "Email Notifications" from the "Helpdesk Productivity" section.
Select the "Requester Notifications" tab and click on "Agent Solves the Ticket".
Paste the snippet you copied into the message editing box. Don't worry if the images look broken, this is normal at this stage.
Save the template.
Check that the notification is turned on with the green check button at the top right above the editing window.