When grouping crew by the aircraft (3-dot filter setting), it is possible to assign crew provisionally.

Right next to aircraft registration click 3 dots and click 'Assign temporary crew'.

In a pop-up window insert: 'Start of assigment', 'End of assigment' and select crew member/members.

In 'Crew Calendar' page, when date range selected in calendar covers dates of assigment, Leon will show crew assigned provisionally in itallic.