Q: I would like to use my new iMac 2015 as external display for my Dell Laptop

Use your iMac as a display with Target Display Mode - Apple Support

This question has been asked many many times, your Dell does not have the ability to be used for TDM. Sorry but that's the simplicity of it. Please read the post mentioned above and it will clearly state that.

Target Display Mode only works between Macs.

You cannot use your iMac as a display for you Dell. Sorry.

I use this app

https://avatron.com/applications/air-display/

to use my iPad as display they have versions which works with computer too

but doubt the performance is good enough for games or movies though