Crop a video to 2.35:1, the CinemaScope frame. A 1920x1080 source exports at 1920x818.
2.35, 2.39, and 2.40
2.35:1 is the original CinemaScope figure. Theatrical practice moved to 2.39:1, usually rounded to 2.40:1 in conversation, and that is what most modern features are finished at. On a 1920 px wide frame the gap is about 14 px of height, which is invisible on its own and matters only when cutting against material finished at the other number.
Two ways to get black bars
These produce different files, and the distinction is worth getting right.
Crop to fill cuts the top and bottom away. A 1920x1080 frame becomes 1920x818 and 262 px of picture is gone. The file is genuinely 2.35:1, and a player letterboxes it on a 16:9 screen using its own bars.
Fit with bars keeps the 16:9 shape and pads the picture into it. Nothing is lost, but the bars are stored as pixels and travel with the file forever.
Which one to pick
Crop when the export is a deliverable and you want the true frame. Fit when the bars are a look and the clip has to stay a standard shape for a platform that expects one.
The second choice has a cost: baked-in bars cannot be removed by a player, and a platform that letterboxes again will stack a second set on top.