Questions answered in this article:
Where can I use the Leadcamp add-in?
Good news!
The add-in can be used both on the web version of Outlook as in the desktop app!
I'm using Microsoft Outlook on my desktop, can I get the Leadcamp add-in?
You can use the Leadcamp add-in with your Microsoft Outlook desktop app. However, "New Outlook" doesn't fully support all functionalities. For example, it's not possible to put library-links behind text. The core functionalities will work though.

How do I pin the Leadcamp add-in for Microsoft Outlook (web version)?
Compose a new email message, select three dots at the bottom of your email and select the Leadcamp add-in.

Pin the add-in to your view.

How do I pin the Leadcamp add-in for Microsoft Outlook (desktop version)?
Compose a new email, scroll to the very end of the top menu in the message window and select the Leadcamp add-in.

Pin the add-in to your view, it will be pinned for all new emails you create.

To pin Leadcamp add-in for Inbox and Sent folders repeat the steps above.