You can do this by doing the following:
1) Go to Outlook Online. Go to the top right and click the PC Download Icon > Install:
It will install Outlook (PWA) on your desktop.
2) Then go to Default Apps in Start Menu:
3) Change your default Email program to Outlook (PWA)
4) Try the mailto feature on a website > Select Remember this choice > press Allow:
It will now open in Outlook PWA (which is your Exchange email) with P1 license.
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