There are two types of background maps: Images() and Boundaries(). Each of these takes a parameter, which is the name of the map to use. The name is one of the maps listed in the window.
For Images(), the choices are Simple Earth, Detailed Earth, NASA, Street Map Service, and Web Map Service. If you use Web Map Service, then there are two additional parameters: the WMS URL and the layer supported by the WMS server.
For Boundaries(), the choices vary since boundaries can be user-defined. A typical choice is World.
dt = Open( "$SAMPLE_DATA/Air Traffic.jmp" );
dt << Graph Builder(
	Size( 1101, 603 ),
	Show Control Panel( 0 ),
	Variables( X( :Longitude ), Y( :Latitude ) ),
	Elements( Points( X, Y, Legend( 8 ) ) ),
	SendToReport(
		Dispatch(
			{},
			"Graph Builder",
			Frame Box,
			{Background Map( Images( "Simple Earth" ), Boundaries( "World" ) ),
			Grid Line Order( 3 ), Reference Line Order( 4 )}
		)
	)
);
JSL Scripting Example shows an excerpt from the map:
JSL Scripting Example
Background Map ( Images ( "Web Map Service", "http://sedac.ciesin.columbia.edu/geoserver/wms", "gpw-v3:gpw-v3-population-density_2000" ), Boundaries ( "US States" ) )
To see which layers are available on a WMS server, install the WMS Explorer Add-In. Download the add-in from the JMP File Exchange at https://community.jmp.com/docs/DOC-6095. Note that some WMS servers are not reliable. If the server is down, or if the user does not have an Internet connection, the WMS map does not appear.
Tips: 
Look at sample data to find examples of background map scripts. Select Help > Sample Data Library and open Napoleons March.jmp, Pollutants Map.jmp, or San Francisco Crime.jmp. Right-click the map table script and select Edit to see the Background Map() function.