FilFlex Add-in can be published to your Exchange by your admin or can be added by yourself using a custom add-in. 

In both methods, the manifest URL needs to be copied and entered while adding the add-in. 

Locating Manifest URL  

Log to FileFlex with any supported browser or native desktop client ( as an end user and not FileFlex admin) . 

Go to Settings →General as shown below. 

The manifest URL is shown as below. Microsoft has removed the option of adding by URL ( shown in the next steps). So we need to download the manifest file by clicking "Download" button.   

Adding using custom Add-in

In your Outlook client click on Get Add-in from the toolbar. 

Click on "My add-ins" from the sidebar and then select "Custom Addins "  

From dropdown, select "Add from File"  ( "Add from URL" has been disabled by Microsoft for all add-ins. ) 

Locate the path of the manifest file saved in previous steps and click next. 

Click "Install" once the warning is displayed. 

This completes your adding of FIleFlex Add-in. 

Adding in Admin managed provider. 

Your admin may have already made the add-in available in your environment. In those cases follow the below steps. 

In your Outlook client click on Get Add-in from the toolbar. 

Click on "Admin-managed" from the sidebar.  

You should locate the FileFlex tile on the subsequent page and click "Add" button. 

 This will add the FileFlex add-in to your Outlook client.