You’re supposed to first create a polygon group ID, which is a list of vertex indices. Check out FPolygonGroupID FMeshDescription::CreatePolygonGroup()
The exposed API is in MeshDescriptionBase.h, but the implementation is in MeshDescription.h. So, you’re supposed to create polygon groups, then create polygons using vertex index arrays, and then you can work on them. It looks to me as if the vertex indices are input reference arguments in the C++ code, yet the Blueprint exports them as outputs, which won’t work – this might be an untested workflow/function at the Blueprint level! (I’d expect the input arguments to be declared const in the C++ code.)
In general, the way this level of code is supposed to be used, is to drop down to C++. Blueprint is great for “when the player pulls the switch, start the elevator.” For lower-level data, you really want to use the C++ API. It’s quite simple to click the “File” menu and “Add C++ Class,” even if your project didn’t start out as a C++ project. Similarly, you don’t have to start the editor from Visual Studio; there’s a “build” button in the editor once it has a C++ class added. (But you still use Visual Studio, or Visual Studio Code, or Ryder, or vim, for actually editing the code.)