In Outlook on the web, you can tag words or phrases in email messages and calendar events to indicate a link to more information about the topic.
Add tags to email messages or meeting requests
Type a hashtag # symbol in the Subject line or the body of an email message or calendar event, and without adding any spaces, enter the name of the tag. A list appears containing tags that already exist, choose from that list, or create a new tag.
Type the message you want that person or group to receive.
Finish creating your email message or calendar event, and click Send.