Crop a video to 21:9, the ultrawide frame. A 1920x1080 source exports at 1920x822.
What 21:9 actually is
The digital ultrawide standard is 64:27, roughly 2.37:1. That sits between CinemaScope at 2.35:1 and modern anamorphic practice at 2.39:1, and 21:9 is the name the monitor industry gave it. The difference against true 2.35:1 is about 5 px of height on a 1920 px wide frame.
Cropping wider costs height
Every crop from 16:9 to something wider takes the top and bottom rather than the sides. A 1920x1080 frame keeps all 1920 columns and becomes 1920x822, losing 258 px of height.
Drag the box up or down before exporting. The default is centred, which is rarely where a horizon or an eyeline sits.
Removing baked-in bars
An ultrawide clip is often delivered inside a 16:9 container with black bars encoded into the picture. Those bars are pixels, not metadata, so a player cannot strip them and a second letterbox stacks on top of the first.
Crop them off here. Set the ratio to 21:9, drag the top and bottom edges of the box in until they sit on the first row of real picture, and export. The result is the frame that was always there, without the border.