Here you can see the question ("Select room"). Click on the plus icon to add the alternatives related to query 1 (in this case "Building").
OBSERVE! The first time you are adding an alternative you cannot click on the plus icon. Then you have to right click the Question ("Select room") and select New.The same window will open as if you use the plus icon.
Go back to Queries to add another query ("Floor").
To make the form more accessible you can make a more detailed description of the field. This description will be used as title in the source code.
Click on Alternative. Highlight an alternative, e.g. "High" and add another alternative ("First" for Floor) under "High" by using the plus icon. Add all alternatives for floors connected to the high building and then do the same thing with the low building.
Go back to Queries. Click the plus icon to add another query ("Room").
Go back once more to Alternative to add the different Rooms. Highlight a floor in the high building and click the plus icon to add the rooms on that floor. Add all alternatives for all floors in both the high and the low buidling
A tip for accessibility! Using title to describe the input field makes it more accessible.