How do I get selected items from Listview control to use as a lookup for additional data

have a look on this

you will have to write a javascript that will parse out data from list view control and prepare them for further usage in form (eg. write them to a text box).

so you will need to put javascript to form's settings custom javascript, or javascript include if you place the code into an external file.

note, the code on the link I provided is just starting point - it will return array of selected items. as written above you will need to parse each array item for data content you need. that's however very specific to used list view content, so it's not possible to provide any generic script.