When grouping crew by the aircraft in 3-dot filter 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 (either start or end date), Leon will show crew assigned provisionally in itallic.

Adding duties for crew assigned provisionally works in the same way as for regular crew.