There's an easy workaround for the problem you are having, What you basically need to do is to provide a technology with which both JavaScript and Blue Prism can interact so that you can bridge the gap.
the easiest thing that comes to mind is an HTML text box.
with JavaScript can create and write to a temporary, invisible textbox on the page and Blue Prism can spy it and read from it.
You can then spy or manually add the element into application modeler