what do i need to allow my macbook pro to run my imac as a second monitor

Only iMacs up to 2014 models can support Target Display Mode. Your model does not support TDM so it cannot be used as an external monitor for your MBP. See Use your iMac as a display with Target Display Mode - Apple Support.

AirParrot and similar applications are used to stream data from one device to another via Wi-Fi or BlueTooth, neither of which is fast enough to act as a mirrored display as you want to do. There are even hardware solutions that use the USB port but they, too, are slow.

Your best solution is to get an external monitor or just use your iMac when working at home.

Yes, you can definitely mirror to an iMac.

While I agree with Kappy about the possibility of it not being smooth enough, AirParrot claims it is now better than Airplay, and as long as your network is good enough it should not be too bad. But only you can determine whether the results work for you.

The 2017 21.5" model does not support Target Display Mode sorry.

iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can’t be used as Target Display Mode displays.

Not that I'm aware of. Perhaps a search with Google might generate alternatives, but my understanding is that later models no longer have the ability to process video input.

There are third party Apps like AirParrot that can do it: https://www.airsquirrels.com/airparrot/features/mac