When generating a mesh via FRawMesh, how to set custom normals?

[anonymous_user_2fccd4ee1 (anonymous_user_2fccd4ee)]

I can set VertexPositions, but no normals.
The closest I found was WedgeTangentZ, which is described as “Normal. Array[WedgeId] = float3(x,y,z)”.
But it doesn’t affect the normals.