Here is a previous thread about the dimensions and aspect ratio used for 21:9 that will go in the custom Video Mode box referenced by @MusicalBox:
If the source footage is 16:9, then some scaling or cropping will have to be done to get the footage to fit in 21:9. The following post has some tricks to make that process easier, although “Crop: Source” is also a good option not mentioned there:
You are right about 21:9 being a gray area. Its dimensions are only used by recent computer monitors. Cinema and television do not use those dimensions, nor do cameras natively record in those dimensions. Cinema and Blu-ray dimensions are listed in the Aspect Ratio section of the following post:
For personal videos, it doesn’t matter which dimensions are used. But if there is any chance of a theatrical, television, or streaming release, then there are very specific constraints imposed by DCI, Netflix, etc. and none currently allow 21:9.