Northern Trail Outfitters wants to include a 'view online' link in an email template to...

The correct personalization string to provide a "View as Webpage" link in an email is %%view_email_url%%. When this string is placed inside the href attribute of a hyperlink, Marketing Cloud automatically generates a URL to a hosted version of the email content.

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Exact Extract from Salesforce Documentation:

Source: Salesforce Help – Personalization Strings

→ "%%view_email_url%% – This personalization string inserts a link to view the email as a webpage. Typically used for the 'View Online' functionality in email templates."

Source: Salesforce Marketing Cloud Email Studio Guide

→ "Use %%view_email_url%% within your hyperlink to allow recipients to view the message as a webpage."