polygon to crop point cloud using the 'crop' function

Hi there,

I was wondering whats the optimum way to crop a point cloud is based on a polygon shape I define with respect to the original point cloud itself.
I.e, I need to do instance object separation on a point cloud, but it requires a json file mentioning the corner points and edge length, but how do I generate this json file wrt to the original point cloud that I am working itself
or is there an even better way to crop without generating the json file

Thanks in advance :)