If an asset is set as the current asset, any subsequent nodes you create are automatically added to that asset.
You can also -click an asset in the Hypergraph Connections editor and select Set Asset as Current.
You can set the heads-up display to show your current asset in the bottom right corner of the scene panel by selecting Display > Heads Up Display > Current Asset.
Default nodes (such as ikSolvers and UI-related nodes) cannot be placed into an asset even when the asset is set as current.