The right thing to do is to replace the lower triple clamp, even if it's a bit of a job (you need to remove the forks but you can't use a headlift stand). Be careful that without that screw, the fork tube will become your steering stop and will eventually damage wires and brake/clutch lines by squeezing them against the frame.
Very likely it's the photo, but the broken part doesn't seem to exactly match what's left on the triple clamp. If it does, then you can try - as a temporary solution - to glue it back with JB Weld. Take your time properly prepping and cleaning the surfaces, but if you do it well it might actually work.
2015 Ducati 899 Panigale