This is what I have in my notes for making O365 work when authenticating as 1 user but wanting to send as another:
In O365 Admin Center, go to 'From' Email user, Mail Tab, Send On Behalf, Add Permissions, Select Authenticating User & Save.
Wait 60 mn to propagate.
Is this what you were looking for?
N
------------------------------
Nancy Hanson
Blytheco LLC
Eagan MN
------------------------------