Add an asset to a material if it does not already have one of the same type.
(Add Asset) to display the Add Asset drop-down menu, and then select the asset type that you want to add. Assets that are already present in the material cannot be added and therefore cannot be selected in the Add Asset drop-down menu.

The Asset Browser opens. The list of assets displayed in the Asset Browser is filtered based on the selected asset type.
button to the right of the asset.
The selected asset is added to the material and displays in the Material Editor panel of the Material Browser dialog.