Go to Restrictions > Height 
You can add height restrictions manually to the network. You first click on the "Height Restriction" menu item.
When you click on the link you want to apply the height restriction you will be prompted to enter a value (0 - 25.5 meters).
When you click OK the restriction is shown on the map.
You should note that the restriction is applied to the whole link and not just the place that you clicked.
The restriction is applied immediately but they are only permanently applied to the network, if you save the restrictions.