We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/sending-from-aliases/259
To set up an alias in Mailspring, go to Preferences > Accounts, choose the proper account, and click “+” at the bottom of the list of aliases. Enter the name and email address you want to send from.
All of your aliases are available in a dropdown when you compose a new message.