how how can I add black bars to video 1920x800 to be 1920x1080 with ffmpeg

I have video 1920x800 without black bars and I need add black bars to put subtitles there. Video should 1920x1080p. The problem is using movavi video has worse quality than original. a lot of worse quality. I put same features from mediainfo of original video. I heard about ffmpeg can edit videos without re-encoding. Therefore I would like if you can help me with my problem I only installed ffmpeg. need to know all thanks