It may not be immediately obvious how to get to the ‘From’ field in Office 365 webmail so you can send as another account or distribution group you have access to, but here’s how to do it.
Open a new email and, if it’s not there already, add the From field by clicking the three little dots and selecting ‘Show From’.
Right-click on the from email address and select the edit option.
You can now use the backspace key to delete the existing email address (don’t worry, it won’t delete the address itself, just allow you to type a new one in its place.)
Type the new address or select it from the address book.