The quick and lazy way of putting a faux-cinemascope black bars in Resolve is by doing this...
Timeline>output blanking>2.39:1 But I will suggest that if you want to always have the bars displayed correctly and the aspect ratio to be correct, watch this on why you shouldn't use Faux black bars and how to do it the right way.
With Resolve, here's how to do it...
MSI Raider X99 MB / Intel i7 5280 6 core CPU / 2 x AMD R9 390X 8Gb GPU / 32Gb 3200 RAM / Decklink 4K Mini Monitor / 240Gb SSD OS Drive / 4Tb RAID 0 SSD Cache/Scratch Drive / 24Tb 7200rpm Data Drive / Win 10 Pro 64bit / Resolve 17