Loads of guides say that installing Virtualbox Guest Additions in your brand new Kali VM is really easy and straightforward, but they always seem to miss a step at the end which means your shared clipboard and drag and drop functionality doesn’t work properly.
TL;DR – You need to call the VBoxClient utility inside the guest with the –clipboard argument, and again with the –draganddrop option as well. Consider adding a third call with –display if you’re struggling to get the autoscaling to work properly.