Outlook Desktop / Send emails via Non-Default Account

Hi, 

How to make non-default account to appear automatically in From field then external application is composing new email. 

Have 2 account in Outlook Desktop 365:

test1 [@] contoso [.] com - default

test2shared [@] contoso [.] com - shared non-default mailbox

Option "Always use the default account when composing new messages" is unchecked already. 

How to make test2shared account to appear automatically in From field without selecting it in drop down list every time we compose a new email or external application is composing new email.