How can I create a block /block reference by selecting the polylines in the drawing
I believe there is a node with a name like Block.ByObjects in the Civil3D toolkit.
What is under block extensions?
Are you using the most up to date version of the toolkit?
If not you can pull the geometry of the object, use a List.Deconstruct to pull the first geometry item out of the list, generate a block from that geometry, and use the new block as the block input for another object creation node.