I am working on an Outlook VSTO project where I need to display emails as HTML within a web form. However, images referenced by CID are not displaying correctly. To resolve this, I downloaded the images to temporary files.

So, I recently developed an Outlook Add-In. The Add-In displays data from an external system, for which an API call is made. However, I was looking to host the add-in. One thing could be via a Web App. But how do I securely store the API key without compromising anything?