Outlook Web App doesn't have the capability to start a new calendar entry when you're reading an email - at least not in an obvious way. You can switch to Calendar and do it there, of course, but who wants to go to such lengths?
Here's how you do it the sneaky way:
Select the message in Outlook Web App, then click on the sender's name to bring up their contact card:
Once the contact card is up, click on the Event button:
This will open up a new calendar entry, with the person's name already entered in the attendees box.
Fill out the rest of the details and send your appointment on its way!