Introduction
In the Control Panel, click Mail > Show Profiles. Then, choose Prompt for a profile to be used and click OK.
Instructions
2) In the upper right corner, next to "View by", choose Large icons. Then, click Mail.
3) Click Show Profiles.
4) Choose the radio button next to "Prompt for a profile to be used." Then click OK.
Outcome
Outlook will now prompt for the profile upon startup.