Usually, in OWA, the Phishing Report Button should always appear in the upper right corner of the message.
If the button is not displayed there, open the settings by clicking on the gear wheel in the upper right corner. Select "View all Outlook settings".
In the window that now opens, select the "Mail" tab, then "Customize actions" and make sure the box for "SoSafe Phishing-Reporting" under "Message Interface" is checked. Then save the settings.