ArcGIS GeoBIM licenses are currently an inclusion of ArcGIS Online ArcGIS Professional Standard and ArcGIS Professional Advanced user types. You can read more on user types here.
If your organization does not have an ArcGIS Professional Standard or Advanced user type, GeoBIM can be purchased as an add-in license and then assigned to a compatible User Type, within your ArcGIS Online subscription. You can see more on how to manage licenses here.
If you would like to learn more about GeoBIM, please feel free to contact us