In order to create boundary conditions in a polygonal region the following workflow can be used:
Extract surface using ExtractSurface (a: Original subsurface mesh; b: Extracted surface)
Mark the mesh elements within the polygonal region deploying the tool ResetPropertiesInPolygonalRegions
Remove marked/unmarked mesh elements deploying tool removeMeshElements
Compute the associated area for the nodes of the surface mesh deploying tool ComputeNodeAreasFromSurfaceMesh
The surface mesh patches (created until step 3) can be used as input for OGS-6 simulations. For OGS-5 simulations only the additional step 4 has to be performed.