Need help, how to cut this in QuickSurface

Hello ,
I am trying to do reverse engineering on a model i scanned, but i am having trouble with part i marked with arrow.

Can someone help me with the method i would do this cut ?

I am using QuickSurface

https://preview.redd.it/ud6ad80s2n9e1.jpg?width=3198&format=pjpg&auto=webp&s=efa99dd75d8ad461bb5d1670f157884b3360eb85