Select portion of mesh

I’m a newbie of Dynamo. I would like to separate a portion from the entire mesh of a facade. I started from a region which I transformed in a curve and extruded towards the facade. I would like to keep only the intersection between the mesh and the extruded curve
I need to use the meshtoolkit for the mesh (as it is quite heavy, made by many triangles). I tried different approaches (intersect, project, split), but the problem is the difficulty (for me) to work with the mesh.
Which is the best approach?
thanks in advance for any suggestions

I used Revit>STL-export>Meshmixer>Dynamo>Revit as workflow when i did it a few years back.
STL-export was a free plugin.

Thanks. To describe the general work, i want to realise some regions in Revit and use them to cut the entire mesh. I need to keep the single portions inside Revit to calculare areas and so on

no any suggestion? as in the image, I would like to separate from the entire mesh of the facade, the part that in inside the grey tube.
thanks in advance