i set this up for my fastmail account a couple weeks ago. go to settings -> aliases and create an alias for * this also allows you to write from any email addr

i set this up for my fastmail account a couple weeks ago. go to settings -> aliases and create an alias for * this also allows you to write from any email address in your domain (when composing, change the sender dropdown to your *-alias, and the 'from' line appears and is editable