In the piping editor, you specify the elevation of the first node with Alt+G. You can also access this function under the "Edit" menu, and in the toolbar.

You may alternatively also specify wind vs elevation in the wind load editor in a few different ways.

Edit to add:

By default the first element is 0,0,0. When you specify wind loads in the input editor the first time, it asks you where you want the origin to be.