Short answer
HubSpot's KB: if the Outlook desktop add-in asks for login each time Outlook opens on Windows, change your internet configuration on the desktop. Office 365 add-in users follow a different troubleshooting path.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Work through it with your IT team if browser settings are managed.
3. Related
Outlook add-in setup and identity errors.