In Outlook 365 go to the Settings button on the toolbar ribbon in Outlook 365, in the upper-right corner. It looks like a small gear. Click on the gear to enter the settings menu, then look on the lower right-hand menu for the option to View all Outlook Settings.
When you are in the all settings screen, look over in the left-hand menu and choose MAIL -> CUSTOMIZE ACTIONS. Check the box next to Report Message. In the window above the checked options, you should see a small icon resembling a white envelope with a small gold dot appear next to the LIKE, REPLY, REPLY ALL and FORWARD buttons. Check and un-check the box once or twice to be sure you see this icon appear in the example window.
Click on Save to be sure this change takes effect and then close the settings menu.
The new icon will only appear inside emails along with the Reply/Forward options, and should be just like it was in the example window when configuring this option. When you click on Report Message icon from within an email message, a drop-down menu appears with several options. Specify which category the email fits, whether it is junk, not junk, or phishing (scam or spam).
In the standalone version of Outlook, the Report Message button should appear automatically on the toolbar ribbon. Be sure you are on the Home tab and you should see the button on the far right of the toolbar.
This button is the same as the button in Outlook 365, with the same options for designating categories of various emails. Choose the appropriate option and send.