This can be done, but I assume you’re aware that some serious cropping is going to happen on the FHD source.
The thread below will provide the steps. You’ll have to create a custom video mode using Settings > Video Mode > Custom > Add and use the following values:
Resolution: 2048 x 858
Aspect ratio: 1024 x 429
Frame rate: 24 for true cinema, but use whatever your source file is
Scan mode: Progressive
Colorspace: Probably BT.709, but use whatever your source file is
Ideally, the video mode would be made first and then video would be imported. It can get interesting making the video in 16:9 first and then converting to scope because the coordinate system for all the filter locations gets changed without proper scaling.
Once past that hurdle, the thread below will describe the zoom factor you’ll need to apply to make a 16:9 video fill a 2.39:1 space. Your zoom factor would be (2048 * 1080) / (1920 * 858) = 134.3%.
The dimensions I gave above would create a true cinema scope resolution, but it would mean upscaling your FHD source from 1920 to 2048, which will make the picture a little blurry for no good reason. To get the same aspect ratio without upscaling (basically doing a crop), use these numbers instead:
Resolution: 1920 x 804
Aspect ratio: 480 x 201
Zoom factor: 134.4%