Using a Late 2015/2016 iMac as a Monitor

I have a late 2015/2016 27" Imac with retina screen. I need to re-locate my headless Home Server currently attached to my TV adjacent to my iMac in the study and although will be accessing the system via Remote desktop there are times I need to connect an external monitor for configuration/maintenance purposes.

I found this website https://itstillworks.com/use-imac-pc-monitor-9320.html that states you can use an iMac as an external monitor by feeding a signal into the mini display port on the iMac.

Before I fork out for a PC video card with Display Port out can anyone confirm that this will actually work in practice?

Many thanks

John