In Outlook for Web, the Phish Alert Button will appear in the actions menu at the top of each email, as shown below:
 

You can report any email as a phishing email. The reported email will be in your Sent Items as a forwarded message and will be deleted from your inbox.  If you incorrectly reported the email, you can retrieve it from the Deleted items/Trash folder.

If you do not see the Phish Alert Button on your menu, it may have gotten disabled, to enable:
Click on the SETTINGS wheel in the upper right, click on MAIL, then CUSTOMIZE ACTIONS.  Make sure there is a check next to Phish Alert then click SAVE:
 

As an option to using the Phish Alert button, you may just forward the phishing email you want to report to [email protected]
This will create a helpdesk ticket automatically.