Replace Survey_Unique_ID_here with your Unique Id which will be found in a survey
For the above Primailer Survey: the Unique ID is yr101apa4z7e9
Step 2: Now add this code to your email template on a button or a text as you like as following:
Set the protocol as other and past [SURVEY:SURVEY_UNIQUE_ID_HERE:VIEW_URL] replacing Survey Unique Id here to your survey unique ID
Step 3: Click save and send a test mail.
The Test/Button will redirect you to the Survey URL.