When creating a new email, View as Web Page is not enabled. If you enable it and clone the email, this setting will be copied.
1. Select your email and click Edit Draft.
2. In the email editor, click Email Settings.
3. Check the Include View as Web Page box and click Save.
Here's an example of how it looks:
You will not see the View as Web Page link until you send the email. Send yourself a test to view.
To change the default text, see Edit the "View as Web Page" Message.