As you can see in the image below taken directly from the U2715H page,
the Apple proprietary operating system and hardware were not tested/validated by Dell. All we can offer is best guess support. You should also post on the Apple Support Communities and see if they can offer some insight into how to extend the Apple OS to the U2715H.
In theory, you would use the U2715H mDP to DP cable =
MacBook Pro 2015 Thunderbolt 2 out port --> U2715H mDP to DP cable --> U2715H DP in port
Open the U2715H OSD (On Screen Display) page 34, choose Input Source- DP