How do I setup Office 365 email different from ID in Win10 Mail App

My Office 365 email address is different from my Office 365 user ID. This is because of the user convention in AD. My Use id is {first letter of first name}{last name}@{domain}, while my primary email address is {first name}.{last name)}@{domain}. When I add an account to Windows 10 Mail App, I start with my email address, but it ends up with my user ID. Therefore I am unable to send from my primary email address, emails got sent from my ID (which is also valid email address associated with my mailbox). How do I send from my primary email address?

I can't help you with your question... but I can say that the Win10 mail app is dreadful. If you've ever used a real email client like Thunderbird or Outlook... Win10's email app is like going back to 4th grade after being in college. OK, admittedly bad analogy. I had to abandon Windows Mail 2012 because MS made some changes to it's mail system. They finally took place over the weekend... at least for me, they've talked about this happening during the summer. I simply could not stand that app, and just installed Thunderbird yesterday.