On the 1D Urban Build tab, initially confirm you are in 1D Urban selection mode by ensuring the toggle is coloured blue.
Click the Rain Gage icon and it will be highlighted. Then on the map view, click the location you would like to add the Rain Gage. You can add the node anywhere - you will specify in your Subcatchment nodes which Rain Gage should be used.
The node will automatically be added to the Nodes tab of the 1D Urban Network panel. As the Multiple checkbox is ticked by default, continue to click the map in more locations to add multiple Rain Gage nodes. Click the Rain Gage icon again to de-select when you have finished adding the nodes.