Install the Protected Trust OWA Add-in for Outlook Web Access in Office 365

To send secure messages from Outlook.com, also known as Office 365 Outlook Web Access (OWA), you may install the Outlook.com Add-in.

The Add-in improves your Outlook experience by adding additional functionality, including:

Quickly toggle between protected and standard messages when composing messages in Outlook
Advanced settings to improve workflows to increase your efficiency

Tip: We highly recommend using Single Sign-On (SSO) for O365 to authenticate, click here to see setup instructions

In order to manually install the Send It Secure add-in for Outlook on the Web, you will first need to download a custom file. Visit the link located at
https://oa.protectedtrust.com/manifest.xml, then right-click on the page and select Save As. Save the file as "manifest.xml" somewhere that will be easy for you to locate in step 2, such as your Desktop.
Visit https://aka.ms/olksideload, which will open Outlook on the Web and automatically open the Add-Ins for Outlook dialog after a few seconds.
Note: This step would replace selecting the Get add-ins icon which may be missing in the most recent versions of Outlook.com
Select My add-ins from the menu on the left
You may notice that the Add from URL.. option is disabled. You will want to use the Add from File... option.
Select + Add a custom add-in > Add from File..

And browse to the file saved during step 3 (Example: Downloads/manifest.xml), and select Open.
Click Install

Optimize the Outlook Add-in Interface
Follow the steps below to optimize the Outlook (OWA) Add-in interface.
Tip: We highly recommend using Single Sign-On (SSO) for O365 to authenticate, click here see setup instructions
1. For best results be sure to take full advantage of the custom action features:
  1. From the top right menu, of any message, select the Protected Trust shield icon
  2. Select the pin icon to pin the Protected Trust Menu for easy use.