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.
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The manifest URL is shown as below and should be copied to be used in subsequent steps.
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. 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
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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. )
Enter Locate the path of the manifest URL copied file saved in earlier previous steps hereand click next. 
Click "Install" once the warning is displayed.
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